Semiconductor device, power diode, and rectifier

ABSTRACT

An object is to provide a semiconductor device having electrical characteristics such as high withstand voltage, low reverse saturation current, and high on-state current. In particular, an object is to provide a power diode and a rectifier which include non-linear elements. An embodiment of the present invention is a semiconductor device including a first electrode, a gate insulating layer covering the first electrode, an oxide semiconductor layer in contact with the gate insulating layer and overlapping with the first electrode, a pair of second electrodes covering end portions of the oxide semiconductor layer, an insulating layer covering the pair of second electrodes and the oxide semiconductor layer, and a third electrode in contact with the insulating layer and between the pair of second electrodes. The pair of second electrodes are in contact with end surfaces of the oxide semiconductor layer.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The technical field of the present invention relates to a semiconductordevice using an oxide semiconductor.

In this specification, the semiconductor device refers to all thedevices that operate by utilizing semiconductor characteristics. In thisspecification, a transistor is included in a non-linear element, thenon-linear element is a semiconductor device, and an electroopticdevice, a semiconductor circuit, and an electronic appliance includingthe non-linear element are all included in semiconductor devices.

2. Description of the Related Art

Many of transistors included in display devices typified by flat paneldisplays (e.g., liquid crystal display devices and light-emittingdisplay devices) include silicon semiconductors such as amorphoussilicon or polycrystalline silicon and are formed over glass substrates.

Attention has been directed to a technique by which, instead of suchsilicon semiconductors, metal oxides exhibiting semiconductorcharacteristics are used for transistors. Note that in thisspecification, a metal oxide exhibiting semiconductor characteristics isreferred to as an oxide semiconductor.

As the oxide semiconductor, a single-component metal oxide such astungsten oxide, tin oxide, indium oxide, or zinc oxide and anIn—Ga—Zn-based oxide semiconductor which is a homologous compound aregiven. Techniques by which a transistor formed using the metal oxide isapplied to a switching element of a pixel in a display device or thelike have been already disclosed in Patent Documents 1 and 2.

As a semiconductor device formed using a silicon semiconductor, there isa semiconductor device for high power application, such as afield-effect transistor including metal and an oxide insulating film (ametal-oxide silicon field-effect transistor: MOSFET), a junctionfield-effect transistor (JFET), and a Schottky barrier diode.

In particular, silicon carbide (SiC), which is a silicon-basedsemiconductor material, is used in a Schottky barrier diode having smallreverse saturation current and excellent withstand voltagecharacteristics (see Patent Document 3).

REFERENCE Patent Document

-   [Patent Document 1] Japanese Published Patent Application No.    2007-123861-   [Patent Document 2] Japanese Published Patent Application No.    2007-096055-   [Patent Document 3] Japanese Published Patent Application No.    2000-133819

SUMMARY OF THE INVENTION

A semiconductor device for high power application needs variouselectrical characteristics such as high withstand voltage, low reversesaturation current, and high on-state current, but so many problemsarise when a semiconductor device having such electrical characteristicsis actually manufactured.

For example, silicon carbide has problems in that it is difficult toobtain a crystal with good quality and a process temperature formanufacturing a semiconductor device is high. For example, an ionimplantation method is used to form an impurity region in siliconcarbide; in that case, heat treatment at 1500° C. or higher is necessaryin order to repair crystal defects caused by ion implantation.

In addition, since carbon is contained, there is a problem in that aninsulating layer with good quality cannot be formed by thermaloxidation. Furthermore, silicon carbide is chemically very stable and isnot easily etched by normal wet etching.

In addition, since the semiconductor device for high power applicationgenerates heat when a large current flows therethrough, a structure thatallows thermal dissipation is needed for the semiconductor device forhigh power application.

Thus, in view of the above problems, an object of one embodiment of thepresent invention is to provide a semiconductor device having electricalcharacteristics such as high withstand voltage, low reverse saturationcurrent, and high on-state current. In particular, an object of anembodiment of the present invention is to provide a power diode and arectifier which include non-linear elements.

An embodiment of the present invention is a semiconductor deviceincluding a first electrode, a gate insulating layer covering the firstelectrode, an oxide semiconductor layer overlapping with the firstelectrode and in contact with the gate insulating layer, a pair ofsecond electrodes covering end portions of the oxide semiconductorlayer, an insulating layer covering the pair of second electrodes andthe oxide semiconductor layer, and a third electrode in contact with theinsulating layer and between the pair of second electrodes. The pair ofsecond electrodes are in contact with end surfaces of the oxidesemiconductor layer.

Note that in this specification, an end surface of the oxidesemiconductor includes a top surface and a side surface in the casewhere a surface of the oxide semiconductor layer on the gate insulatinglayer side is referred to as a bottom surface. That is, a pair of secondelectrodes are in contact with the oxide semiconductor layer in a regionexcept a channel formation region and a region in contact with a gateinsulating layer. Therefore, the pair of second electrodes serve as aheat sink, and when heat is generated due to current flowing in theoxide semiconductor layer including the channel formation region, thepair of second electrodes can dissipate the heat to the outside.

Another embodiment of the present invention is in the abovesemiconductor device in which n⁺ layers are provided between the gateinsulating layer and the end portions of the oxide semiconductor layer,and the pair of second electrodes, in order to reduce contact resistancebetween the pair of second electrodes and the oxide semiconductor layer.

A depletion layer in the oxide semiconductor has a large thickness;therefore, in the semiconductor device, high on-state current can beobtained by increasing the thickness of the oxide semiconductor layer.In other words, another embodiment of the present invention is thesemiconductor device in which the thickness of the oxide semiconductorlayer is greater than or equal to 0.1 μm and less than or equal to 50μm, preferably greater than or equal to 0.5 μm and less than or equal to20 μm.

The oxide semiconductor layer may be a crystalline oxide semiconductorlayer. With such a structure, a highly reliable semiconductor device inwhich variation in the electrical characteristics due to irradiationwith visible light or ultraviolet light is suppressed can be achieved.The crystalline oxide semiconductor layer includes an oxide including acrystal with c-axis alignment (also referred to as C-Axis AlignedCrystal (CAAC)), which has neither a single crystal structure nor anamorphous structure. Note that part of the crystalline oxidesemiconductor layer includes crystal grains. In other words, anotherembodiment of the present invention is the above-described semiconductordevice in which the oxide semiconductor layer is a crystalline oxidesemiconductor layer, and the crystalline oxide semiconductor layer hasan a-b plane parallel to a surface of the crystalline oxidesemiconductor layer and has c-axis alignment in a directionperpendicular to the surface.

Another embodiment of the present invention is the above-describedsemiconductor device in which the crystalline oxide semiconductor layerincludes one or both of zinc and indium.

Another embodiment of the present invention is the above-describedsemiconductor device in which the first electrode functions as a gateelectrode, the pair of second electrodes function as a source electrodeand a drain electrode, and the third electrode functions as a back gateelectrode.

As the above-described semiconductor device, a power diode in which aplurality of non-linear elements are connected in series in the forwarddirection is given. In other words, another embodiment of the presentinvention is a power diode including a plurality of non-linear elements.The non-linear element includes a first electrode, a gate insulatinglayer covering the first electrode, an oxide semiconductor layeroverlapping with the first electrode and in contact with the gateinsulating layer, a pair of second electrodes in contact with the oxidesemiconductor layer, an insulating layer covering the pair of secondelectrodes and the oxide semiconductor layer, and a third electrode incontact with the insulating layer and between the pair of secondelectrodes.

The pair of second electrodes are in contact with end surfaces of theoxide semiconductor layer, one of the pair of second electrodes is incontact with the first electrode with the gate insulating layer providedtherebetween, and the non-linear elements are connected in series in aforward direction.

As the above-described semiconductor device, a rectifier including twonon-linear elements having the above-described structure is given. Inother words, another embodiment of the present invention is a rectifierincluding a first non-linear element and a second non-linear element.The first non-linear element and the second non-linear element eachinclude a first electrode, a gate insulating layer covering the firstelectrode, an oxide semiconductor layer overlapping with the firstelectrode and in contact with the gate insulating layer, a pair ofsecond electrodes in contact with the oxide semiconductor layer, aninsulating layer covering the pair of second electrodes and the oxidesemiconductor layer, and third electrode in contact with the insulatinglayer and between the pair of second electrodes. The pair of secondelectrodes are in contact with end surfaces of the oxide semiconductorlayer, one of the pair of second electrodes is in contact with the firstelectrode with the gate insulating layer provided therebetween, an anodeof the first non-linear element is connected to a reference potential ona low potential side, a cathode of the first non-linear element isconnected to an input portion and an anode of the second non-linearelement, and a cathode of the second non-linear element is connected toan output portion.

Further, as the above-described semiconductor device, a rectifierincluding four non-linear elements having the above-described structureis given. In other words, another embodiment of the present invention isa rectifier including first to fourth non-linear elements. The first tofourth non-linear elements each includes first electrode, a gateinsulating layer covering the first electrode, an oxide semiconductorlayer overlapping with the first electrode and in contact with the gateinsulating layer, a pair of second electrodes in contact with the oxidesemiconductor layer, an insulating layer covering the pair of secondelectrodes and the oxide semiconductor layer, and a third electrode incontact with the insulating layer and between the pair of secondelectrodes. The pair of second electrodes are in contact with endsurfaces of the oxide semiconductor layer, one of the pair of secondelectrodes is in contact with the first electrode with the gateinsulating layer provided therebetween. An anode of the first non-linearelement is connected to a reference potential on a low potential sideand a cathode thereof is connected to a first input portion. An anode ofthe second non-linear element is connected to the first input portionand a cathode thereof is connected to an output portion. An anode of thethird non-linear element is connected to a second input portion and acathode thereof is connected to the output portion. An anode of thefourth non-linear element is connected to the reference potential on thelow potential side and a cathode thereof is connected to the secondinput portion.

A semiconductor which has characteristics such as higher withstandvoltage and lower reverse saturation current and can have higheron-state current as compared to a conventional semiconductor device canbe provided. In addition, a semiconductor device in which degradationdue to heat generation can be suppressed can be provided.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIGS. 1A to 1C are a plan view and cross-sectional views illustrating anon-linear element that is one embodiment of the present invention;

FIGS. 2A to 2D are plan views illustrating a method for manufacturing anon-linear element that is one embodiment of the present invention;

FIGS. 3A to 3D are cross-sectional views illustrating a method formanufacturing a non-linear element that is one embodiment of the presentinvention;

FIGS. 4A to 4C are a plan view and cross-sectional views illustrating anon-linear element that is one embodiment of the present invention;

FIGS. 5A to 5C are cross-sectional views illustrating a method formanufacturing a non-linear element that is one embodiment of the presentinvention;

FIGS. 6A and 6B are cross-sectional views illustrating a non-linearelement that is one embodiment of the present invention;

FIGS. 7A and 7B illustrate a two-dimensional crystal;

FIGS. 8A to 8D are cross-sectional views illustrating a method formanufacturing a non-linear element that is one embodiment of the presentinvention;

FIGS. 9A and 9B are cross-sectional views illustrating a non-linearelement that is one embodiment of the present invention;

FIGS. 10A1 and 10A2, FIGS. 10B1 and 10B2, and FIGS. 10C1 to 10C2illustrate power diodes and rectifiers that are embodiments of thepresent invention;

FIGS. 11A and 11B are a plan view and a cross-sectional viewillustrating a diode that is one embodiment of the present invention;

FIG. 12 is a top view of a manufacturing apparatus for manufacturing oneembodiment of the present invention;

FIGS. 13A to 13C are cross-sectional views illustrating a structure of anon-linear element used for calculation;

FIG. 14 is a graph showing a calculation result of a drain current inthe non-linear element illustrated in FIGS. 13A to 13C;

FIGS. 15A to 15C are cross-sectional views illustrating a structure of anon-linear element used for calculation as a comparative example;

FIGS. 16A to 16C are cross-sectional views illustrating a structure of anon-linear element used for calculation as a comparative example;

FIGS. 17A to 17C are cross-sectional views illustrating a structure of anon-linear element used for calculation as a comparative example;

FIG. 18 is a graph showing calculation results of drain currents in thenon-linear elements of FIGS. 13A to 13C and FIGS. 15A to 17C.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, embodiments of the present invention will be described indetail with reference to the accompanying drawings. However, the presentinvention is not limited to the following description and it is easilyunderstood by those skilled in the art that the mode and details can bevariously changed without departing from the scope and spirit of thepresent invention. Accordingly, the invention should not be construed asbeing limited to the description of the embodiments below. In describingstructures of the present invention with reference to the drawings, thesame reference numerals are used in common for the same portions indifferent drawings. The same hatching pattern is applied to similarparts, and the similar parts are not especially denoted by referencenumerals in some cases. Note that the size, the layer thickness, or theregion of each structure illustrated in each drawing is exaggerated forclarity in some cases. Therefore, the present invention is notnecessarily limited to such scales illustrated in the drawings.

In an illustration of a stack of layers (or electrodes) included in atransistor, an end portion of a lower layer which protrudes from an endportion of an upper layer is not illustrated in some cases forconvenience in a plan view of the transistor.

Further, when it is described that “A and B are connected to eachother”, the case where A and B are electrically connected to each otherand the case where A and B are directly connected to each other areincluded. Here, each of A and B corresponds to an object (e.g., adevice, an element, a circuit, a wiring, an electrode, a terminal, aconductive film, or a layer).

Note that a voltage refers to a difference between potentials of twopoints, and a potential refers to electrostatic energy (electricpotential energy) of a unit charge at a given point in an electrostaticfield. Note that in general, a difference between a potential of onepoint and a reference potential is merely called a potential or avoltage, and a potential and a voltage are used as synonymous words inmany cases. Thus, in this specification, a potential may be rephrased asa voltage and a voltage may be rephrased as a potential unless otherwisespecified.

Note that, functions of “source” and “drain” may become switched in thecase that a direction of a current flow is changed during circuitoperation, for example. Therefore, the terms “source” and “drain” can beused to denote the drain and the source, respectively, in thisspecification.

On-state current refers to a current (a drain current) which flowsbetween a source electrode and a drain electrode when a transistor ison. For example, in the case of an n-channel transistor, on-statecurrent refers to a drain current when a gate voltage of the transistoris higher than a threshold voltage of the transistor. Off-state currentrefers to a current (a drain current) which flows between a sourceelectrode and a drain electrode when a transistor is off. For example,in an n-channel transistor, off-state current is a drain current when agate voltage is lower than a threshold voltage of the transistor.Further, an on/off ratio refers to the ratio of on-state current tooff-state current.

In this specification, an n-channel transistor whose the thresholdvoltage is positive is defined as a normally-off transistor, while ap-channel transistor whose threshold voltage is negative is defined as anormally-off transistor. Further, an n-channel transistor whosethreshold voltage is negative is defined as a normally-on transistor,while a p-channel transistor whose threshold voltage is positive isdefined as a normally-on transistor.

Embodiment 1

In this embodiment, a structure of a non-linear element that is oneembodiment of the present invention and a method for manufacturing thenon-linear element will be described with reference to FIGS. 1A to 1C,FIGS. 2A to 2D, and FIGS. 3A to 3D. Note that a transistor is describedas an example in this embodiment.

FIG. 1A is a plan view of a transistor 100, and FIG. 1B is across-sectional view taken along line A-B in the transistor 100. FIG. 1Cis a cross-sectional view taken along line C-D in the transistor 100.Note that a base insulating layer 102, a gate insulating layer 105, andan insulating layer 111 are not illustrated in FIG. 1A for convenience.FIG. 1A illustrates a wiring 104 including a first electrode 103functioning as a gate electrode; an oxide semiconductor layer 107including a channel formation region; a wiring 110 including a pair ofsecond electrodes 109 a and 109 b functioning as a source electrode anda drain electrode; and a wiring 114 provided between the pair of secondelectrodes 109 a and 109 b, overlapping with the oxide semiconductorlayer 107 with the insulating layer 111 provided therebetween, andincluding a third electrode 113 serving as a back gate electrode.Although the first electrode 103, the pair of second electrodes 109 aand 109 b, and the third electrode 113 are integrated with the wiring104, the wiring 110, and the wiring 114, respectively here, theelectrodes and the wirings may be formed separately and electricallyconnected to each other.

In this embodiment, although the first electrode 103, the pair of secondelectrodes 109 a and 109 b, and the third electrode 113 function as agate electrode, a source and a drain electrode, and a back gateelectrode, respectively, one embodiment of the present invention is notlimited thereto. The first electrode 103, the pair of second electrodes109 a and 109 b, and the third electrode 113 each can function as any ofa gate electrode, a source electrode, a drain electrode, and a back gateelectrode.

As illustrated in FIG. 1B, the transistor 100 is a dual-gate transistorincluding a gate electrode and a back gate electrode and includes thebase insulating layer 102, the first electrode 103, the gate insulatinglayer 105, the oxide semiconductor layer 107, the pair of secondelectrodes 109 a and 109 b, the insulating layer 111, and the thirdelectrode 113, which are provided over a substrate 101.

The first electrode 103 is provided in contact with the base insulatinglayer 102. The gate insulating layer 105 is provided to cover the firstelectrode 103. The oxide semiconductor layer 107 is provided in contactwith the gate insulating layer 105 to overlap with the first electrode103. The pair of second electrodes 109 a and 109 b cover end portions ofthe oxide semiconductor layer 107. The insulating layer 111 covers partof the oxide semiconductor layer 107 and the pair of second electrodes109 a and 109 b. The third electrode 113 is provided on and in contactwith the insulating layer 111 and between the pair of second electrodes109 a and 109 b.

Further, since the pair of second electrodes 109 a and 109 b cover theend portions of the oxide semiconductor layer 107, the pair of secondelectrodes 109 a and 109 b are in contact with end surfaces of the oxidesemiconductor layer 107. Therefore, in a region where the pair of secondelectrodes 109 a and 109 b are in contact with the oxide semiconductorlayer 107, the width of each of the pair of second electrodes 109 a and109 b is larger than the width of a channel formed in the oxidesemiconductor layer 107 (see FIG. 1A).

Further, as illustrated in FIG. 1C, since the pair of second electrodes109 a and 109 b are in contact with the end surfaces of the oxidesemiconductor layer 107, the pair of second electrodes 109 a and 109 bserve as a heat sink, and when heat is generated due to high on-statecurrent flowing in the oxide semiconductor layer 107, the pair of secondelectrodes 109 a and 109 b can dissipate the heat to the outside. As aresult, degradation of the transistor 100 due to heat generation can besuppressed.

In addition, in the transistor 100, the width of the first electrode 103is larger than the width of the oxide semiconductor layer 107 in thechannel length direction, the third electrode 113 overlaps with theoxide semiconductor layer 107 with the insulating layer 111 providedtherebetween, and the width of the third electrode 113 is at leastlarger than or equal to the channel length.

Accordingly, end portions of the oxide semiconductor layer 107 which arenot covered with the pair of the second electrodes 109 a and 109 b arecovered with the first electrode 103 and the third electrode 113 withthe gate insulating layer 105 and the insulating layer 111 providedtherebetween. In other words, all of the end portions of the oxidesemiconductor layer 107 are covered with the first electrode 103, thepair of second electrodes 109 a and 109 b, and the third electrode 113.

When the end portions of the oxide semiconductor layer 107 are coveredeven in the case where the gate insulating layer 105 and the insulatinglayer 111 are provided between the first electrode 103 and the oxidesemiconductor layer 107 and between the third electrode 113 and theoxide semiconductor layer 107, the pair of second electrodes 109 a and109 b can serve as a heat sink. Therefore, in the transistor 100, heatwhich is generated when high on-state current flows in the oxidesemiconductor layer 107 can be effectively dissipated to the outside, sothat degradation of the transistor 100 due to heat generation can besuppressed.

As the substrate 101, an alkali-free glass substrate formed with afusion method or a float method, a plastic substrate having heatresistance sufficient to withstand heat treatment performed later, orthe like can be used. In addition, a substrate where an insulating filmis provided on a surface of a metal substrate such as a stainless steelsubstrate, or a substrate where an insulating film is provided on asurface of a semiconductor substrate may be used.

As a glass substrate, if the temperature of the heat treatment to beperformed later is high, a glass substrate whose strain point is 730° C.or higher is preferably used. As a glass substrate, a glass materialsuch as aluminosilicate glass, aluminoborosilicate glass, or bariumborosilicate glass is used, for example. By containing a larger amountof barium oxide (BaO) than boric oxide, a more practical heat-resistantglass substrate is obtained. Therefore, a glass substrate containing BaOand B₂O₃ so that the amount of BaO is larger than that of B₂O₃ ispreferably used.

Note that a substrate formed of an insulator, such as a ceramicsubstrate, a quartz substrate, or a sapphire substrate, may be usedinstead of the glass substrate. Alternatively, crystallized glass or thelike may be used.

The base insulating layer 102 provided between the substrate 101 and thefirst electrode 103 can prevent not only diffusion of an impurityelement from the substrate 101 but also etching of the substrate 101during an etching step included in the steps for manufacturing thetransistor. Therefore, the thickness of the base insulating layer 102 ispreferably, but not limited to, 50 nm or more. Note that the baseinsulating layer 102 is formed with a single-layer structure or astacked structure using an oxide insulator and/or a nitride insulatorsuch as silicon oxide, gallium oxide, aluminum oxide, silicon nitride,silicon oxynitride, aluminum oxynitride, and silicon nitride oxide. Inparticular, aluminum nitride, aluminum nitride oxide, and siliconnitride which have a high thermal conductivity are effective inimproving thermal dissipation when used for the base insulating layer102. In addition, alkali metal such as Li and Na is an impurity for theoxide semiconductor layer 107 described later. Therefore, it ispreferable to reduce the content of alkali metal. In the case where aglass substrate including an impurity such as alkali metal is used asthe substrate 101, the base insulating layer 102 is preferably formedusing a nitride insulator such as silicon nitride or aluminum nitride inorder to prevent the entry of the alkali metal.

The first electrode 103 serving as a gate electrode can be formed with asingle-layer structure or a stacked structure using a metal materialsuch as molybdenum, titanium, tantalum, tungsten, aluminum, copper,chromium, neodymium, or scandium or an alloy material which includes anyof these materials as its main component. In addition, the firstelectrode 103 can have a single-layer structure or a stacked structureincluding two or more layers. For example, a single-layer structure ofan aluminum film containing silicon, a two-layer structure of analuminum film and a titanium film stacked thereover, a two-layerstructure of a tungsten film and a titanium film stacked thereover, athree-layer structure in which a titanium film, an aluminum film, and atitanium film are stacked in this order, and the like can be given.

There is no limitation on the thickness of the first electrode 103, andthe thickness of the first electrode 103 can be determined asappropriate considering the electrical resistance of a conductive filmformed using any of the above-described materials and a time necessaryfor forming the conductive film.

The gate insulating layer 105 is in contact with the oxide semiconductorlayer 107 and thus needs to have high quality. This is because the oxidesemiconductor layer 107 which is an i-type or a substantially i-typeoxide semiconductor layer obtained by removal of impurities (an oxidesemiconductor layer whose hydrogen concentration is reduced and to whichoxygen is supplied) is extremely sensitive to an interface state andinterface electric charge, and thus an interface between the oxidesemiconductor layer 107 the gate insulating layer 105 is important.

The gate insulating layer 105 can be formed using any of theabove-described oxide insulators. A portion of the gate insulating layer105 which is in contact with the oxide semiconductor layer 107preferably contains oxygen, and, in particular, the gate insulatinglayer 105 preferably contains oxygen whose amount exceeds thestoichiometric proportion. For example, the gate insulating layer 105may be formed using silicon oxide (SiO_(2+α) (note that α>0)) whichcontains oxygen whose amount exceeds the stoichiometric proportion. Byforming the gate insulating layer 105 with the use of the silicon oxide,part of oxygen contained in the gate insulating layer 105 can besupplied to the oxide semiconductor layer 107 in the heat treatmentperformed in the manufacturing steps of the transistor 100, so that thetransistor 100 can have favorable electrical characteristics.

The gate insulating layer 105 may be formed with either a single-layerstructure or a stacked structure. When the thickness of the gateinsulating layer 105 is increased, gate leakage current can be reduced.When the gate insulating layer 105 is formed using, for example, ahigh-k material such as hafnium oxide, yttrium oxide, hafnium silicate(HfSi_(x)O_(y) (x>0, y>0)), hafnium silicate to which nitrogen is added(HfSiO_(x)N_(y) (x>0, y>0)), or hafnium aluminate (HfAl_(x)O_(y) (x>0,y>0)), gate leakage current can be reduced. Note that the thickness ofthe gate insulating layer is preferably greater than or equal to 50 nmand less than or equal to 500 nm.

The oxide semiconductor layer 107 can be formed using any of thefollowing: a four-component metal oxide such as an In—Sn—Ga—Zn-basedmetal oxide; a three-component metal oxide such as an In—Ga—Zn-basedmetal oxide, an In—Sn—Zn-based metal oxide, an In—Al—Zn-based metaloxide, a Sn—Ga—Zn-based metal oxide, an Al—Ga—Zn-based metal oxide, aSn—Al—Zn-based metal oxide, an In—Hf—Zn-based metal oxide, anIn—La—Zn-based metal oxide, an In—Ce—Zn-based metal oxide, anIn—Pr—Zn-based metal oxide, an In—Nd—Zn-based metal oxide, anIn—Sm—Zn-based metal oxide, an In—Eu—Zn-based metal oxide, anIn—Gd—Zn-based metal oxide, an In—Tb—Zn-based metal oxide, anIn—Dy—Zn-based metal oxide, an In—Ho—Zn-based metal oxide, anIn—Er—Zn-based metal oxide, an In—Tm—Zn-based metal oxide, anIn—Yb—Zn-based metal oxide, or an In—Lu—Zn-based metal oxide; atwo-component metal oxide such as an In—Zn-based metal oxide, aSn—Zn-based metal oxide, an Al—Zn-based metal oxide, a Zn—Mg-based metaloxide, a Sn—Mg-based metal oxide, an In—Mg-based metal oxide, or anIn—Ga-based metal oxide; or a single-component metal oxide containingindium, tin, zinc or the like. The oxide semiconductor layer 107including the channel region is preferably formed using the metal oxidecontaining zinc or the metal oxide containing zinc and indium,considering the manufacture of a crystalline oxide semiconductordescribed later. Here, for example, an In—Ga—Zn-based metal oxide meansan oxide containing indium (In), gallium (Ga), and zinc (Zn), and thereis no particular limitation on the composition ratio thereof. Further,the In—Ga—Zn-based metal oxide may contain an element other than In, Ga,and Zn.

In addition, it is preferable that impurities such as hydrogen besufficiently removed from the oxide semiconductor layer 107 and oxygenbe sufficiently supplied thereto. Specifically, the hydrogenconcentration of the oxide semiconductor layer 107 is 5×10¹⁹ atoms/cm³or lower, preferably 5×10¹⁸ atoms/cm³ or lower, more preferably 5×10¹⁷atoms/cm³ or lower. Note that the hydrogen concentration of the oxidesemiconductor layer 107 is measured by secondary ion mass spectroscopy(SIMS). By supply of sufficient oxygen, a defect level due to oxygendeficiency in the energy gap of the oxide semiconductor layer 107 isreduced. Accordingly, a carrier density of the oxide semiconductor layer107 due to a donor such as hydrogen is higher than or equal to 1×10¹⁰cm⁻³ and lower than or equal to 1×10¹³ cm⁻³. In this manner, thetransistor 100 with extremely favorable off-state currentcharacteristics can be obtained with the use of an i-type (intrinsic) orsubstantially i-type oxide semiconductor for the oxide semiconductorlayer 107. For example, the off-state current (per unit channel width (1μm) here) at room temperature (25° C.) is 100 zA (1 zA (zeptoampere) is1×10⁻²¹ A) or less, preferably 10 zA or less.

In addition, the content of alkali metal such as Li and Na is preferablylow, and the concentration of alkali metal in the oxide semiconductorlayer 107 is preferably 2×10¹⁶ cm⁻³ or lower, preferably 1×10¹⁵ cm⁻³ orlower. Further the content of alkaline earth metal is preferably lowbecause alkaline earth metal is also an impurity. The reason isdescribed below. Note that it has been pointed out that an oxidesemiconductor is insensitive to impurities, there is no problem when aconsiderable amount of metal impurities is contained in the film, andtherefore, soda-lime glass which contains a large amount of alkali metalsuch as sodium (Na) and is inexpensive can also be used (Kamiya, Nomura,and Hosono, “Carrier Transport Properties and Electronic Structures ofAmorphous Oxide Semiconductors: The present status”, KOTAI BUTSURI(SOLID STATE PHYSICS), 2009, Vol. 44, pp. 621-633). But suchconsideration is not appropriate. Alkali metal is not an elementincluded in an oxide semiconductor, and therefore, is an impurity. Also,alkaline earth metal is impurity in the case where alkaline earth metalis not included in an oxide semiconductor. Alkali metal, in particular,Na becomes Na⁺ when an insulating film in contact with the oxidesemiconductor layer is an oxide and Na diffuses into the insulatingfilm. Further, in the oxide semiconductor layer, Na cuts or enters abond between metal and oxygen which are included in an oxidesemiconductor. As a result, degradation of the electricalcharacteristics occurs; for example, the field-effect mobility isreduced or the transistor becomes a normally-on transistor in which adrain current flows even in the state where no voltage is applied to thegate electrode (Vg=0), which is caused by the shift of the thresholdvoltage in the negative direction. In addition, variation in theelectrical characteristics also occurs. Such degradation of electricalcharacteristics of the transistor and variation in the electricalcharacteristics due to the impurities remarkably appear when thehydrogen concentration in the oxide semiconductor layer is sufficientlylow.

An In—Ga—Zn-based metal oxide has a sufficiently high resistance when noelectric field is applied thereto and thus can sufficiently reduceoff-state current. Further, an In—Ga—Zn-based metal oxide has highfield-effect mobility and is therefore a preferable semiconductormaterial for the transistor of one embodiment of the present invention.

A depletion layer is thicker in the case of using an oxide semiconductorfor a channel formation region like the transistor 100, as compared tothe case of using a silicon semiconductor for a channel formationregion. Accordingly, the channel region is thicker because the channelregion is also formed in a depth direction of the oxide semiconductor.Further, when the thickness of the oxide semiconductor is large, a largenumber of carriers can flow. As a result, high on-state current can beobtained.

The drain withstand voltage of the transistor 100 depends on thethickness of the oxide semiconductor layer 107. Therefore, in order toincrease the drain withstand voltage, the thickness of the oxidesemiconductor layer 107 is preferably large and may be selected inaccordance with the desired drain withstand voltage.

Accordingly, the thickness of the oxide semiconductor layer 107 ispreferably greater than or equal to 0.1 μm and less than or equal to 50μm, preferably greater than or equal to 0.5 μm and less than or equal to20 μm in consideration of the electrical characteristics such as theon-state current and the drain withstand voltage.

The drain withstand voltage of a transistor using an oxide semiconductoris now described.

When the electric field in the semiconductor reaches a certain thresholdvalue, impact ionization occurs, carriers accelerated by the highelectric field impact crystal lattices in a depletion layer, therebygenerating pairs of electrons and holes. When the electric field becomeseven higher, the pairs of electrons and holes generated by the impactionization are further accelerated by the electric field, and the impactionization is repeated, resulting in an avalanche breakdown in whichcurrent is increased exponentially. The impact ionization occurs becausecarriers (electrons and holes) have kinetic energy that is larger thanor equal to the band gap of the semiconductor. It is known that theimpact ionization coefficient that shows probability of impactionization has correlation with the band gap and that the impactionization is unlikely to occur as the band gap is increased.

Since the band gap of the oxide semiconductor is about 3.15 eV, which islarger than the band gap of silicon, i.e., about 1.12 eV, the avalanchebreakdown is unlikely to occur. Therefore, a transistor using the oxidesemiconductor has a high drain withstand voltage, and an exponentialsudden increase of on-state current is expected to be unlikely to occurwhen a high electric field is applied.

Next, hot-carrier degradation of a transistor using an oxidesemiconductor is described.

The hot-carrier degradation means deterioration of transistorcharacteristics, e.g., shift in the threshold voltage or gate leakagecurrent, which is caused as follows: electrons that are accelerated tobe rapid are injected in the vicinity of a drain in a channel into agate insulating film and become fixed electric charge or form traplevels at the interface between the gate insulating film and the oxidesemiconductor. The factors of the hot-carrier degradation are, forexample, channel-hot-electron injection (CHE injection) anddrain-avalanche-hot-carrier injection (DAHC injection).

Since the band gap of a silicon semiconductor is narrow, electrons arelikely to be generated like an avalanche owing to an avalanchebreakdown, and electrons that are accelerated to be so rapid as to goover a barrier to the gate insulating film are increased in number.However, the oxide semiconductor described in this embodiment has a wideband gap; therefore, the avalanche breakdown is unlikely to occur andresistance to the hot-carrier degradation is higher than that of asilicon semiconductor. In this manner, the transistor including an oxidesemiconductor has high drain withstand voltage, and the transistorincluding an oxide semiconductor is suitable for a semiconductor devicefor high power application such as an insulated-gate field effecttransistor (IGFET), a junction field-effect transistor, and a Schottkybarrier diode.

The pair of second electrodes 109 a and 109 b can be formed using any ofthe materials given in the description of the first electrode 103. Thethickness and structure of the electrodes are appropriately selectedbased on the description of the first electrode 103. Note that since thepair of second electrodes 109 a and 109 b function as a heat sink whichdissipates heat generated when the on-state current flows in the oxidesemiconductor layer 107 to the outside, the pair of second electrodes109 a and 109 b are preferably formed using a metal material or an alloymaterial which easily conducts heat.

The insulating layer 111 can be formed using any of the oxide insulatorsgiven in the description of the gate insulating layer 105. Since theinsulating layer 111 is also in contact with the oxide semiconductorlayer 107, a portion of the insulating layer 111 which is in contactwith the oxide semiconductor layer 107 preferably includes oxygen, andthe insulating layer 111 is preferably formed using silicon oxide(SiO_(2+α) (note that α>0)) which contains oxygen whose amount exceedsthe stoichiometric proportion, in particular. By forming the insulatinglayer 111 with the use of the silicon oxide, part of oxygen contained inthe insulating layer 111 can be supplied to the oxide semiconductorlayer 107 in the heat treatment performed in the manufacturing steps ofthe transistor 100, so that the transistor 100 can have favorableelectrical characteristics. In addition, the insulating layer 111 may beformed using the high-k material given in the description of the gateinsulating layer 105. Further the insulating layer 111 may be formedwith either a single-layer structure or a stacked structure. When thethickness of the insulating layer 111 is increased, gate leakage currenton the back gate side can be reduced. The thickness of the insulatinglayer 111 is preferably greater than or equal to 50 nm and less than orequal to 500 nm.

The third electrode 113 serving as a back gate electrode can be formedusing any of the materials given in the description of the firstelectrode 103, and the thickness and structure of the third electrode113 may be appropriately selected based on the description of the firstelectrode 103.

In view of reliability, the electrical characteristics of the transistorincluding an oxide semiconductor are changed by irradiation with visiblelight or ultraviolet light or application of heat or an electric field.As an example of a change of the electrical characteristics, thetransistor becomes a normally-on transistor in which the drain currentflows even in the state where no voltage is applied to the gateelectrode (Vg=0). In the case of an n-channel transistor, in which anelectron is a majority carrier, an electron in the drain current flowsin a region where a depletion layer is formed. Therefore, in thetransistor, the region where an electron flows includes a region in thevicinity of a top surface of the oxide semiconductor layer 107, wherethe pair of second electrodes 109 a and 109 b and the insulating layer111 are provided. Therefore, a hole is induced in the insulating layer111 in contact with the oxide semiconductor layer 107 (in particular, ina region in the vicinity of a bottom surface of the insulating layer 111in contact with the oxide semiconductor layer 107) and the transistorbecomes normally-on as time passes. Thus, since the transistor describedin this embodiment is a dual-gate transistor including the thirdelectrode 113, voltage can be freely applied to the third electrode 113,and the threshold voltage (Vth) can be controlled, so that thetransistor can be prevented from becoming normally-on.

In addition, since the transistor described in this embodiment is adual-gate transistor, voltage can be applied to the third electrode 113,and a channel can be efficiently formed even when the thickness of theoxide semiconductor layer 107 is large, so that high on-state currentcan be obtained.

Here, the shape of the third electrode 113 is described with referenceto FIGS. 2A to 2D.

The third electrode 113 illustrated in FIG. 2A has the same shape as thethird electrode 113 illustrated in FIG. 1A. The third electrode 113 isprovided to be parallel to the first electrode 103 and overlaps with thepair of second electrodes 109 a and 109 b with the insulating layer 111provided therebetween. In that case, voltage applied to the thirdelectrode 113 and voltage applied to the first electrode 103 can becontrolled independently.

The third electrode 113 illustrated in FIG. 2B is parallel to the firstelectrode 103 but does not overlap with the pair of second electrodes109 a and 109 b. Also in this structure, the voltage applied to thethird electrode 113 and the voltage applied to the first electrode 103can be controlled independently.

The third electrode 113 illustrated in FIG. 2C can be connected to thefirst electrode 103. In other words, the first electrode 103 and thethird electrode 113 are connected to each other in an opening portion150 formed in the gate insulating layer 105 and the insulating layer111. In this structure, the voltage applied to the third electrode 113is equal to the voltage applied to the first electrode 103.

Further, a structure illustrated in FIG. 2D may be employed, in whichthe third electrode 113 is not connected to the first electrode 103 butis in a floating state.

In addition, in the structures illustrated in FIGS. 2C and 2D, the thirdelectrode 113 may overlap with the pair of second electrodes 109 a and109 b with the insulating layer 111 provided therebetween.

Although not illustrated in FIGS. 1A to 1C, a protective insulatinglayer may be provided over the insulating layer 111 and the thirdelectrode 113 in the transistor 100.

Next, a method for manufacturing the transistor 100 is described withreference to FIGS. 3A to 3D.

The base insulating layer 102 is formed over the substrate 101. Throughthis process, impurities in a glass substrate can be prevented fromentering the transistor which is to be formed.

The base insulating layer 102 can be formed by a sputtering method, aCVD method, a coating method, or the like. In this embodiment, a siliconoxide film is formed by a sputtering method with the use of a silicontarget. In order to remove moisture and hydrogen in the base insulatinglayer 102, the substrate 101 may be subjected to heat treatment afterthe base insulating layer 102 is formed.

Next, the first electrode 103 is formed over the base insulating layer102. Note that the step for forming the first electrode 103 is combinedwith a step for forming the wiring 104 (see FIG. 1A). The firstelectrode 103 can be formed in such a manner that a conductive film isformed over the substrate 101 by a sputtering method, a vacuumevaporation method, or a CVD method, a resist mask is formed over theconductive film by a first photolithography step, and the conductivefilm is etched using the resist mask. Alternatively, the resist mask isformed by a printing method or an ink-jet method instead of thephotolithography step, so that the number of steps for forming the firstelectrode 103 can be reduced. Note that end portions of the firstelectrode 103 preferably have a tapered shape because the coverage withthe gate insulating layer 105 formed later can be improved. Note thatthe taper shape can be obtained by etching while the resist mask is madeto recede.

In this embodiment, a conductive film (e.g., a tungsten film) with athickness of 150 nm is formed by a sputtering method and etched with theuse of a resist mask formed through the first photolithography step, sothat the first electrode 103 is formed. Note that the etching step withthe use of the resist mask includes a step for removing the resist maskeven when its description is not expressly made in this specification.

Next, the gate insulating layer 105 covering the first electrode 103 isformed. The gate insulating layer 105 is in contact with the oxidesemiconductor layer 107 which is to be formed later, and thus, needs tohave high quality. When the oxide semiconductor layer 107 is in contactwith the gate insulating layer 105 having high quality, the interfacestate density between the oxide semiconductor layer 107 and the gateinsulating layer 105 is reduced and the interface characteristics becomefavorable. As a result, the transistor 100 completed can have favorableelectrical characteristics.

The gate insulating layer 105 can be formed by any of the methods givenin the description of the base insulating layer 102. In this embodiment,silicon oxide (SiO_(2+α) (note that α>0)) containing oxygen whose amountexceeds the stoichiometric proportion is formed as the gate insulatinglayer 105. Note that the silicon oxide formed has a thickness of 200 nm.

When the silicon oxide is formed by a sputtering method, a silicontarget or a quartz target is used as a target and oxygen or a mixed gasof oxygen and argon is used as a sputtering gas. At that time, the gateinsulating layer 105 is preferably formed while hydrogen, moisture,hydroxyl groups, hydride, or the like remaining in a process chamber areremoved. An entrapment vacuum pump is preferably used in order to removehydrogen, water, hydroxyl groups, hydride, or the like remaining in theprocess chamber. As the entrapment vacuum pump, for example, a cryopump,an ion pump, or a titanium sublimation pump is preferably used. Theevacuation unit may be a turbo pump provided with a cold trap. Hydrogen,water, hydroxyl groups, hydride, or the like is removed from the processchamber which is evacuated with a cryopump; thus, when the gateinsulating layer 105 is formed in the process chamber, the concentrationof hydrogen, water, hydroxyl groups, or hydride contained in the gateinsulating layer 105 can be reduced.

It is preferable to use a high-purity gas from which an impurity such ashydrogen, water, a hydroxyl group, or hydride is removed to aconcentration of several ppm or several ppb, as a sputtering gas usedwhen the gate insulating layer 105 is formed.

In this embodiment, the substrate 101 is transferred to the processchamber, a sputtering gas containing high-purity oxygen, from whichhydrogen, water, hydroxyl groups, hydride, or the like is removed, isintroduced, and the silicon oxide is formed as the gate insulating layer105 over the substrate 101 using a silicon target. Note that the gateinsulating layer 105 may be formed while the substrate 101 is heated.

In addition, in the case where the gate insulating layer 105 is formedwith a stacked structure, for example, silicon nitride may be formedbetween the silicon oxide and the substrate 101. The silicon nitride isformed using a silicon target and sputtering gas containing high-puritynitrogen from which hydrogen, water, hydroxyl groups, hydride, or thelike is removed. Further it is preferable that the silicon nitride beformed while hydrogen, water, hydroxyl groups, hydride, or the likeremaining in the process chamber is removed in a manner similar to thecase of the silicon oxide.

In the case where the silicon nitride and the silicon oxide are stackedto form the gate insulating layer 105, the silicon nitride and thesilicon oxide can be formed in the same process chamber with the samesilicon target. In that case, a sputtering gas containing nitrogen isintroduced and silicon nitride is formed using a silicon target mountedin the process chamber, and then, the sputtering gas is switched to asputtering gas containing oxygen and the same silicon target is used toform silicon oxide. The silicon nitride and the silicon oxide can besuccessively formed by this method without exposure to the air, wherebyhydrogen, water, a hydroxyl group, hydride, or the like can be preventedfrom being adsorbed on a surface of the silicon nitride.

Further, preheat treatment is preferably performed before the gateinsulating layer 105 is formed, in order to remove hydrogen, water,hydroxyl groups, or hydride which remains on the inner wall of theprocess chamber, on a surface of the target, or inside the targetmaterial. After the preheat treatment, the substrate 101 or the processchamber is cooled and then the gate insulating layer 105 is formedwithout exposure to the air. In that case, not water but oil or the likeis preferably used as a coolant for the target.

Further, in the case where the gate insulating layer 105 is formed by aCVD method, for example, by a high-density plasma CVD with the use of amicrowave (e.g., its frequency is 2.45 GHz), the gate insulating layerwhich is dense and has high withstand voltage and high quality can beformed. In addition, since the gate insulating layer formed byhigh-density plasma CVD can have a uniform thickness, the gateinsulating layer has excellent step coverage. Further, as for the gateinsulating layer formed using high-density plasma CVD, the thickness canbe controlled precisely.

Next, an oxide semiconductor film 106 is formed to overlap with thefirst electrode 103 with the gate insulating layer 105 providedtherebetween. The oxide semiconductor film 106 can be formed over thegate insulating layer 105 by a sputtering method, a molecular beamepitaxy method, an atomic layer deposition method, a pulsed laserdeposition method, a coating method, or a printing method.

In this embodiment, the oxide semiconductor film 106 is formed by asputtering method. The oxide semiconductor film 106 is formed over thegate insulating layer 105 in such a manner that a sputtering gas fromwhich hydrogen, water, hydroxyl groups, hydride, or the like is removedis introduced into the process chamber and a metal oxide is used as atarget while the substrate is held in the process chamber held in areduced-pressure state and moisture remaining in the process chamber isremoved. Hydrogen, water, hydroxyl groups, hydride, or the likeremaining in the process chamber may be removed in a manner similar tothe gate insulating layer 105. Consequently, impurities such ashydrogen, water, hydroxyl groups, or hydride, (and preferably a compoundcontaining a carbon atom) are exhausted from the process chamber, sothat the concentration of the impurities contained in the oxidesemiconductor film 106 can be reduced. Further, the oxide semiconductorfilm 106 may be formed while the substrate 101 is heated.

As the target used for forming the oxide semiconductor film 106 by asputtering method, a metal oxide target containing at least zinc or ametal oxide target containing at least zinc and indium can be used. Inthis embodiment, the oxide semiconductor film 106 with a thickness of500 nm is deposited using an In—Ga—Zn-based metal oxide target(In₂O₃:Ga₂O₃:ZnO=1:1:2 [molar ratio]). As other examples of the metaloxide target, a target having a composition of In₂O₃:Ga₂O₃:ZnO=1:1:1[molar ratio], a target having a composition of In:Ga:Zn=1:1:0.5 [atomicratio], a target having a composition of In:Ga:Zn=1:1:1 [atomic ratio],and the like are given. Any of the above-described metal oxide targetsmay contain SiO₂ at 2 wt % or more and 10 wt % or less. Note that thefilling rate of the metal oxide target is greater than or equal to 90%and less than or equal to 100%, preferably greater than or equal to 95%and less than or equal to 99.9%. By using the metal oxide target withhigh filling rate, a dense oxide semiconductor film is formed.

The oxide semiconductor film 106 is formed in a rare gas (typicallyargon) atmosphere, an oxygen atmosphere, or an atmosphere including arare gas (typically argon) and oxygen. It is preferable to use ahigh-purity gas from which an impurity such as hydrogen, water, ahydroxyl group, or hydride is removed to a concentration of several ppmor several ppb, as a sputtering gas used when the oxide semiconductorfilm 106 is formed.

For example, formation conditions are set as follows: the distancebetween the substrate 101 and the target is 170 mm, the substratetemperature is 250° C., the pressure is 0.4 Pa, and the direct current(DC) power is 0.5 kW.

As pretreatment, it is preferable that the substrate 101 subjected tothe steps up to the formation of the gate insulating layer 105 bepreheated and impurities such as hydrogen, water, hydroxyl groups, orhydride adsorbed on the substrate 101 be eliminated and exhausted sothat hydrogen is contained in the oxide semiconductor film 106 as littleas possible. Note that exhaust is preferably performed using a cryopumpat the time of the preheating. Note that this preheating may be omitted.Alternatively, the preheating may be performed on the substrate 101before the first electrode 103 is formed or after the oxidesemiconductor layer 107 is formed later.

Note that before the oxide semiconductor film 106 is formed bysputtering, reverse sputtering in which plasma is generated byintroduction of an argon gas is preferably performed so that dust or anoxide film which is attached to a surface of the gate insulating layer105 is removed, in which case the resistance at an interface between thegate insulating layer 105 and the oxide semiconductor film 106 can bereduced. The reverse sputtering refers to a method of modifying asurface of a substrate by applying a voltage to the substrate using anRF power source in an argon atmosphere to form plasma in a vicinity ofthe substrate. Note that instead of an argon atmosphere, a nitrogenatmosphere, a helium atmosphere, or the like may be used. Alternatively,an argon atmosphere to which oxygen, nitrous oxide, or the like is addedmay be used. Further alternatively, an argon atmosphere to whichchlorine, carbon tetrafluoride, or the like is added may be used.

The structure obtained through the steps up to here is illustrated inFIG. 3A.

Next, the oxide semiconductor film 106 is etched using a resist maskformed through a second photolithography step, so that a firstisland-shaped oxide semiconductor layer is formed.

The step for forming the first island-shaped oxide semiconductor layeris described. The first island-shaped oxide semiconductor layer isformed by etching with the use of the resist mask formed through thesecond photolithography step. The second photolithography step issimilar to the first photolithography step.

For the etching of the oxide semiconductor film 106, either wet etchingor dry etching may be employed. In addition, these may be combined. Asan etchant used for wet etching, a mixed solution of phosphoric acid,acetic acid, and nitric acid, an ammonia hydrogen peroxide mixture(hydrogen peroxide water at 31 wt % ammonia water at 28 wt%:water=5:2:2), or the like can be used. In addition, ITO07N (producedby KANTO CHEMICAL CO., INC.) may also be used.

The etchant after the wet etching is removed together with the etchedmaterial by cleaning. The waste liquid including the etchant and theetched material may be purified and the material may be reused. When amaterial such as indium is collected from the waste liquid after theetching and reused, the resources can be efficiently used and the costcan be reduced.

As the etching gas for dry etching, a gas containing chlorine(chlorine-based gas such as chlorine (Cl₂), boron trichloride (BCl₃),silicon tetrachloride (SiCl₄), or carbon tetrachloride (CCl₄)) ispreferably used.

Alternatively, a gas containing fluorine (fluorine-based gas such ascarbon tetrafluoride (CF₄), sulfur hexafluoride (SF₆), nitrogentrifluoride (NF₃), or trifluoromethane (CHF₃)); hydrogen bromide (HBr);oxygen (O₂); any of these gases to which a rare gas such as helium (He)or argon (Ar) is added; or the like can be used.

As the dry etching method, a parallel plate RIE (reactive ion etching)method or an ICP (inductively coupled plasma) etching method can beused. In order to etch the film into a desired shape, the etchingconditions (the amount of electric power applied to a coil-shapedelectrode, the amount of electric power applied to an electrode on asubstrate side, the temperature of the electrode on the substrate side,or the like) is adjusted as appropriate.

Next, first heat treatment is performed on the first island-shaped oxidesemiconductor layer to form an oxide semiconductor layer 126.

The temperature of the first heat treatment is higher than or equal to400° C. and lower than or equal to 750° C., preferably higher than orequal to 400° C. and lower than the strain point of the substrate 101.Here, the substrate is introduced into an electric furnace that is akind of heat treatment apparatus and heat treatment is performed on thefirst island-shaped oxide semiconductor layer at 450° C. in anatmosphere of an inert gas such as nitrogen or a rare gas for one hour.After that, the oxide semiconductor layer 126 is not exposed to the air;consequently, hydrogen, water, a hydroxyl group, hydride, or the likecan be prevented from entering the oxide semiconductor layer 126. As aresult, the oxide semiconductor layer 126 whose hydrogen concentrationis reduced can be obtained. That is, at least one of dehydration anddehydrogenation of the first island-shaped oxide semiconductor layer canbe performed by this first heat treatment.

Further, in addition to the dehydration and dehydrogenation, the firstheat treatment is also combined with treatment by which, in the casewhere the gate insulating layer 105 contains oxygen, part of the oxygenis diffused into an interface between the gate insulating layer 105 andthe first island-shaped oxide semiconductor layer or the vicinitythereof. As a result of the treatment, the oxide semiconductor layer 126whose oxygen deficiency is reduced can be obtained.

Note that it is preferable that in the first heat treatment, hydrogen,water, a hydroxyl group, hydride, or the like be not contained innitrogen or a rare gas such as helium, neon, or argon. Alternatively,the purity of nitrogen or a rare gas such as helium, neon, or argonwhich is introduced into the heat treatment apparatus is greater than orequal to 6 N (99.9999%), preferably greater than or equal to 7 N(99.99999%) (i.e., the impurity concentration is lower than or equal to1 ppm, preferably lower than or equal to 0.1 ppm).

The heat treatment apparatus used for the first heat treatment is notlimited to a particular apparatus, and the apparatus may be providedwith a device for heating an object to be processed by heat radiation orheat conduction from a heating element such as a resistance heatingelement. For example, an electric furnace, or a rapid thermal annealing(RTA) apparatus such as a gas rapid thermal annealing (GRTA) apparatusor a lamp rapid thermal annealing (LRTA) apparatus can be used. An LRTAapparatus is an apparatus for heating an object to be processed byradiation of light (an electromagnetic wave) emitted from a lamp such asa halogen lamp, a metal halide lamp, a xenon arc lamp, a carbon arclamp, a high pressure sodium lamp, or a high pressure mercury lamp. AGRTA apparatus is an apparatus for heat treatment using ahigh-temperature gas.

Further, the first heat treatment may be performed before the firstisland-shaped oxide semiconductor layer is formed. In other words, thefirst heat treatment may be performed on the oxide semiconductor film106. In that case, the substrate is taken out of the heat treatmentapparatus after the first heat treatment, and then the secondphotolithography step and the etching step are performed.

The structure obtained through the steps up to here is illustrated inFIG. 3B.

Then, a conductive film which is to be processed into the pair of secondelectrodes 109 a and 109 b is formed over the gate insulating layer 105and the oxide semiconductor layer 126. After that, the conductive filmis etched using a resist mask formed through a third photolithographystep, so that the pair of second electrodes 109 a and 109 b are formed.The method for forming the pair of second electrodes 109 a and 109 b maybe similar to the method for forming the first electrode 103. In thisembodiment, the conductive film is formed to a thickness of 150 nm usingtitanium by a sputtering method. Note that the step for forming the pairof second electrodes 109 a and 109 b is combined with a step for formingthe wiring 110 (see FIG. 1A).

The structure obtained through the steps up to here is illustrated inFIG. 3C.

Next, the insulating layer 111 is formed in contact with the pair ofsecond electrodes 109 a and 109 b and part of the oxide semiconductorlayer 126. The method for forming the insulating layer 111 may besimilar to the method for forming the gate insulating layer 105. In thisembodiment, the insulating layer 111 is formed by a sputtering methodwith the use of silicon oxide. Note that the thickness of the insulatinglayer 111 is 200 nm. After that, second heat treatment whose heattemperature is different from that of the first heat treatment isperformed. By the second heat treatment, part of oxygen included in thegate insulating layer 105 and the insulating layer 111 is supplied tothe oxide semiconductor layer 126, so that the oxide semiconductor layer107 is formed. As the heat temperature of the second heat treatment getshigher, the amount of change in the threshold voltage which is caused bylight irradiation or application of a BT stress becomes small. However,when the heat temperature is higher than 320° C., the on-statecharacteristics are degraded. Thus, the second heat treatment isperformed under the conditions that the atmosphere is an inertatmosphere, an oxygen atmosphere, or a mixed atmosphere of oxygen andnitrogen, and the heat temperature is higher than or equal to 200° C.and lower than or equal to 400° C., preferably higher than or equal to250° C. and lower than or equal to 320° C. In addition, heating time ofthe heat treatment is longer than or equal to 1 minute and shorter thanor equal to 24 hours. Note that the second heat treatment may beperformed after the formation of the third electrode 113 to be formedlater. In addition, silicon nitride may be formed over the insulatinglayer 111 to prevent the intrusion of moisture or alkali metal. Sincealkali metal such as Li or Na is an impurity, the content of alkalimetal is preferably reduced. The concentration of the alkali metal inthe oxide semiconductor layer 107 is lower than or equal to 2×10¹⁶ cm⁻³,preferably, lower than or equal to 1×10¹⁵ cm³. Further the content ofalkaline earth metal is preferably low because alkaline earth metal isalso an impurity. Note that after the third electrode 113 describedlater is formed, silicon nitride may be formed as a protectiveinsulating layer. In that case, the following steps and the like areappropriately performed: a step for forming opening portions in theprotective insulating layer; a step for forming a conductive film to beelectrically connected to the first electrode 103, the pair of secondelectrodes 109 a and 109 b, and the third electrode 113.

Next, the third electrode 113 is formed in contact with the insulatinglayer 111 so as to overlap with a channel formation region of the oxidesemiconductor layer 107. The third electrode 113 is formed in such amanner that a conductive film is formed on the insulating layer 111 andthen the conductive film is etched using a resist mask formed through afourth photolithography step. The method for forming the third electrode113 may be similar to the method for forming the first electrode 103. Inthis embodiment, the conductive film is formed to a thickness of 150 nmby a sputtering method with the use of molybdenum. Note that the stepfor forming the third electrode 113 is combined with a step for formingthe wiring 114 (see FIG. 1A).

The structure obtained through the steps up to here is illustrated inFIG. 3D.

Through the above steps, a non-linear element with high withstandvoltage, low reverse saturation current, and high on-state current canbe obtained. Note that this embodiment can be implemented in combinationwith any of the structures described in other embodiments, asappropriate.

Embodiment 2

In this embodiment, a non-linear element whose structure is partlydifferent from that of the non-linear element described in Embodiment 1will be described. Note that a transistor is used as an example also inthis embodiment.

FIG. 4A is a plan view of a transistor 200, and FIG. 4B is across-sectional view taken along line E-F in the transistor 200. FIG. 4Cis a cross-sectional view taken along line G-H in the transistor 200.Note that the transistor 200 is a modified example of the transistor100; therefore, in FIGS. 4A to 4C, the same reference numerals are usedfor the same parts as those in FIGS. 1A to 1C, and detailed descriptionof the same reference numerals is omitted.

As illustrated in FIG. 4B, the transistor 200 is a dual-gate transistorand includes a base insulating layer 102, a first electrode 103, a gateinsulating layer 105, an oxide semiconductor layer 107, n⁺ layers 117 aand 117 b, a pair of second electrodes 109 a and 109 b, an insulatinglayer 111, and a third electrode 113, which are provided over asubstrate 101.

The first electrode 103 is provided in contact with the base insulatinglayer 102. The gate insulating layer 105 is provided to cover the firstelectrode 103. The oxide semiconductor layer 107 is provided in contactwith the gate insulating layer 105 to overlap with the first electrode103. The n⁺ layers 117 a and 117 b are formed to cover the gateinsulating layer 105 and end portions of the oxide semiconductor layer107. The pair of second electrodes 109 a and 109 b are provided over then⁺ layers 117 a and 117 b to cover the end portions of the oxidesemiconductor layer 107. The insulating layer 111 covers part of theoxide semiconductor layer 107 and the pair of second electrodes 109 aand 109 b. The third electrode 113 is provided on and in contact withthe insulating layer 111 and between the pair of second electrodes 109 aand 109 b.

The n⁺ layers 117 a and 117 b are formed between the oxide semiconductorlayer 107 and the pair of second electrodes 109 a and 109 b, wherebycontact resistance between the oxide semiconductor layer 107 and thepair of second electrodes 109 a and 109 b can be reduced. As a result,high on-state current can be obtained. In addition, when the n⁺ layers117 a and 117 b are formed, parasitic capacitance can be reduced and theamount of change in on-state current (Ion degradation) between beforeand after application of a negative gate stress in a BT test can besuppressed.

Although only one of the pair of second electrodes 109 a and 109 b isillustrated in FIG. 1C, an etching step is performed using the pair ofsecond electrodes 109 a and 109 b as masks to form the n⁺ layers 117 aand 117 b, and thus the n⁺ layers 117 a and 117 b are processed so thatthe end portions of the n⁺ layers 117 a and 117 b protrude from the pairof second electrodes 109 a and 109 b. Thus, the channel length of thetransistor 200 is determined by the distance between the n⁺ layer 117 aand the n⁺ layer 117 b. Although the pair of second electrodes 109 a and109 b face end surfaces of the oxide semiconductor layer 107 with the n⁺layers 117 a and 117 b provided therebetween, the pair of secondelectrodes 109 a and 109 b serve as a heat sink in a manner similar toEmbodiment 1 because the n⁺ layers 117 a and 117 b are not formed to beextremely thick and do not block the conduction of heat generated in theoxide semiconductor layer 107. As a result, heat which is generated whenhigh on-state current flows in the oxide semiconductor layer 107 can bedissipated to the outside, so that degradation of the transistor 200 dueto heat generation can be suppressed.

Steps for manufacturing the transistor 200 are described below. Thesteps for manufacturing the transistor 200 are the same as those formanufacturing the transistor 100 except a step for forming the n⁺ layers117 a and 117 b. Therefore, description is made with reference to FIGS.3A and 3B and FIGS. 5A to 5C.

In accordance with the manufacturing steps described in Embodiment 1(after the structure of FIG. 3A is obtained), the structure of FIG. 3Bis obtained.

Next, a film 115 to be the n⁺ layers 117 a and 117 b is formed to athickness greater than or equal to 1 nm and less than or equal to 200 nmover the gate insulating layer 105 and the oxide semiconductor layer 107with the use of an In—Zn-based metal oxide, an In—Sn-based metal oxide,or a single-component metal oxide containing indium or tin. The methodfor forming the film 115 may be the same as the method for forming theoxide semiconductor layer 107. In addition, SiO₂ may be contained in theabove material for the n⁺ layer. In this embodiment, an In—Sn-basedmetal oxide film containing SiO₂ is formed to a thickness of 100 nm.

Next, a conductive film 108 for forming the pair of second electrodes109 a and 109 b is formed over the film 115 which is to be the n⁺ layers(see FIG. 5A). The conductive film is processed to form the pair ofsecond electrodes 109 a and 109 b. Note that the step for forming thepair of second electrodes 109 a and 109 b is combined with a step forforming a wiring 110 (see FIG. 4A).

Then, the film 115 is processed using the pair of second electrodes 109a and 109 b as masks to form the n⁺ layers 117 a and 117 b. Through thisprocessing, the n⁺ layers 117 a and 117 b are formed so that the endportions thereof protrude from the pair of second electrodes 109 a and109 b (see FIG. 5B). Thus, the channel length of the transistor 200 isdetermined by the distance between the n⁺ layer 117 a and the n⁺ layer117 b. On the other hand, the channel length of the transistor 100described in Embodiment 1 is determined by the distance between the pairof second electrodes 109 a and 109 b. It is preferable that the taperangle of each of the end portions of the n⁺ layers 117 a and 117 b (anangle formed by the side surface of either the n⁺ layer 117 a or the n⁺layers 117 b and a planar surface of the substrate 101) be less than orequal to 30°.

The subsequent steps are the same as those of Embodiment 1. Theinsulating layer 111 covering part of the oxide semiconductor layer 107and the pair of second electrodes 109 a and 109 b is formed. Further,the third electrode 113 is formed in contact with the insulating layer111 so as to overlap with a channel formation region of the oxidesemiconductor layer 107 (see FIG. 5C). Details of the steps are the sameas those of Embodiment 1.

Through the above steps, a non-linear element with high withstandvoltage, low reverse saturation current, and high on-state current canbe obtained. Note that this embodiment can be implemented in combinationwith any of the structures described in other embodiments, asappropriate.

Embodiment 3

In this embodiment, a non-linear element whose structure is partlydifferent from that of the non-linear element described in Embodiment 1will be described. Note that a transistor is used as an example also inthis embodiment.

A transistor 300 described in this embodiment is a transistor obtainedin such a manner that the oxide semiconductor layer 107 of thetransistor 100 described in Embodiment 1 is replaced with a crystallineoxide semiconductor stack 120 including a first crystalline oxidesemiconductor layer 107 a and a second crystalline oxide semiconductorlayer 107 b. In short, a plan structure of the transistor 300 is similarto that of the transistor 100; therefore, FIG. 1A can be referred to forthe plan view of the transistor 300. FIG. 6A is a cross-sectional viewtaken along line A-B in the transistor 300. FIG. 6B is a cross-sectionalview taken along line C-D in the transistor 300. Note that thetransistor 300 is a modified example of the transistor 100; therefore,in FIGS. 6A and 6B, the same reference numerals are used for the sameparts as those in FIGS. 1A to 1C, and detailed description of the samereference numerals is omitted.

As illustrated in FIG. 6A, the transistor 300 is a dual-gate transistorand includes a base insulating layer 102, a first electrode 103, a gateinsulating layer 105, the crystalline oxide semiconductor stack 120, apair of second electrodes 109 a and 109 b, an insulating layer 111, anda third electrode 113, which are provided over a substrate 101.

The first electrode 103 is provided in contact with the base insulatinglayer 102. The gate insulating layer 105 is provided to cover the firstelectrode 103. The crystalline oxide semiconductor stack 120 is providedin contact with the gate insulating layer 105 to overlap with the firstelectrode 103. The pair of second electrodes 109 a and 109 b cover endportions of the crystalline oxide semiconductor stack 120. Theinsulating layer 111 covers part of the crystalline oxide semiconductorstack 120 and the pair of second electrodes 109 a and 109 b. The thirdelectrode 113 is provided on and in contact with the insulating layer111 and between the pair of second electrodes 109 a and 109 b.

In addition, since the pair of second electrodes 109 a and 109 b areprovided to cover the end portions of the crystalline oxidesemiconductor stack 120, the pair of second electrodes 109 a and 109 bare in contact with the end surfaces of the crystalline oxidesemiconductor stack 120. Therefore, at least in a region where the pairof second electrodes 109 a and 109 b are in contact with the crystallineoxide semiconductor stack 120, the width of each of the pair of secondelectrodes 109 a and 109 b is larger than the width of a channel formedin the crystalline oxide semiconductor stack 120 (see FIG. 1A).

Further, only one of the pair of second electrodes 109 a and 109 b isillustrated in FIG. 6B. Since the pair of second electrodes 109 a and109 b are in contact with the end surfaces of the crystalline oxidesemiconductor stack 120, the pair of second electrodes 109 a and 109 bserve as a heat sink in a manner similar to other embodiments, and whenheat is generated due to high on-state current flowing in thecrystalline oxide semiconductor stack 120, the pair of second electrodes109 a and 109 b can dissipate the heat to the outside. As a result,degradation of the transistor 300 due to heat generation can besuppressed.

Here, the crystalline oxide semiconductor stack 120 is described. Thecrystalline oxide semiconductor stack 120 has a stacked structure of thefirst crystalline oxide semiconductor layer 107 a and the secondcrystalline oxide semiconductor layer 107 b.

The first crystalline oxide semiconductor layer 107 a has c-axisalignment. In addition, the second crystalline oxide semiconductor layer107 b also has c-axis alignment. Note that the first crystalline oxidesemiconductor layer 107 a and the second crystalline oxide semiconductorlayer 107 b include an oxide including a crystal with c-axis alignment(also referred to as c-axis aligned crystal (CAAC)), which has neither asingle crystal structure nor an amorphous structure.

An oxide including CAAC refers to an oxide including a crystal withc-axis alignment, which has a triangular or hexagonal atomic arrangementwhen seen from the direction of an a-b plane, a surface, or aninterface. In the crystal, metal atoms are arranged in a layered manner,or metal atoms and oxygen atoms are arranged in a layered manner alongthe c-axis, and the direction of the a-axis or the b-axis is varied inthe a-b plane (the crystal rotates around the c-axis).

In a broad sense, an oxide including CAAC means a non-single-crystaloxide including a phase which has a triangular, hexagonal, regulartriangular, or regular hexagonal atomic when seen from the directionperpendicular to the a-b plane and in which metal atoms are arranged ina layered manner or metal atoms and oxygen atoms are arranged in alayered manner when seen from the direction perpendicular to the c-axis.

The CAAC includes a crystalline region (a crystal region) but a boundarybetween one crystal region and another crystal region is not necessarilyclear. That is, the first crystalline oxide semiconductor layer 107 aand the second crystalline oxide semiconductor layer 107 b partlyinclude a crystal grain boundary.

In the case where oxygen is included in the CAAC, nitrogen may besubstituted for part of oxygen included in the CAAC. The c-axes ofindividual crystalline portions included in the CAAC may be aligned inone direction (e.g., a direction perpendicular to a surface of asubstrate over which the CAAC is formed, a surface, a film surface, oran interface of the CAAC). In addition, normals of the a-b planes ofindividual crystal regions included in the CAAC may be aligned in onedirection (e.g., a direction perpendicular to the surface of thesubstrate over which the CAAC is formed, or the surface, the filmsurface, or the interface of the CAAC).

The CAAC may be a conductor, a semiconductor, or an insulator dependingon its composition or the like. Further, The CAAC may transmit or nottransmit visible light depending on its composition or the like.

As an example of such a CAAC, there is a crystal which is formed into afilm shape and has a triangular or hexagonal atomic arrangement whenobserved from the direction perpendicular to a surface of the film or, asurface of a substrate, or an interface and in which metal atoms arearranged in a layered manner or metal atoms and oxygen atoms (ornitrogen atoms) are arranged in a layered manner when a cross section ofthe film is observed.

The first crystalline oxide semiconductor layer 107 a and the secondcrystalline oxide semiconductor layer 107 b are preferably formed usingmetal oxide containing at least zinc or metal oxide containing at leastzinc and indium. For example, among the metal oxides described inEmbodiment 1, an In—Sn—Ga—Zn-based metal oxide which is a four-componentoxide; an In—Ga—Zn-based metal oxide, an In—Sn—Zn-based metal oxide, anIn—Al—Zn-based metal oxide, a Sn—Ga—Zn-based metal oxide, anAl—Ga—Zn-based metal oxide, or a Sn—Al—Zn-based metal oxide, which is athree-component metal oxide; an In—Zn-based metal oxide, a Sn—Zn-basedmetal oxide, or an Al—Zn-based metal oxide which is a two-componentmetal oxide; a single-component metal oxide containing Zn; or the likecan be used.

The first crystalline oxide semiconductor layer 107 a is formed by asputtering method in which a substrate temperature is higher than orequal to 200° C. and lower than or equal to 400° C., and after theformation, first heat treatment (at a temperature higher than or equalto 400° C. and lower than or equal to 750° C.) is performed.

Here, the crystal structure of the CAAC is described. Although dependingon the temperature of the first heat treatment, the first heat treatmentcauses crystallization from a film surface and crystal grows from thefilm surface toward the inside of the film; thus, a c-axis alignedcrystal is obtained. By the first heat treatment, a large amount of zincand oxygen gather to the film surface, and one or more layers ofgraphene-type two-dimensional crystal including zinc and oxygen andhaving a hexagonal upper plane (a schematic plan view thereof is shownin FIG. 7A) are formed at the outermost surface; the layers of crystalat the outermost surface grow in the thickness direction to form a stackof layers. In FIG. 7A, a white circle indicates a zinc atom, and a blackcircuit indicates an oxygen atom. By increasing the temperature of theheat treatment, crystal growth proceeds from the surface to the insideand further from the inside to the bottom. Further, FIG. 7Bschematically shows a stack formed of six layers of two-dimensionalcrystal as an example of a stack in which two-dimensional crystal hasgrown.

In the case where oxygen is contained in the gate insulating layer 105,part of oxygen is diffused into an interface between the gate insulatinglayer 105 and the first crystalline oxide semiconductor layer 107 a orthe vicinity thereof by the first heat treatment, so that oxygendeficiency of the first crystalline oxide semiconductor layer 107 a isreduced. Accordingly, at least oxygen whose amount exceeds thestoichiometric proportion is preferably contained in the film (in thebulk) of the gate insulating layer 105 or the interface between thefirst crystalline oxide semiconductor layer 107 a and the gateinsulating layer 105.

The second crystalline oxide semiconductor layer 107 b is formed by asputtering method in which a substrate temperature in deposition ishigher than or equal to 200° C. and lower than or equal to 400° C. Bysetting the substrate temperature in the deposition to be higher than orequal to 200° C. and lower than or equal to 400° C., precursors can bearranged in the oxide semiconductor layer formed on and in contact withthe surface of the first crystalline oxide semiconductor layer 107 a andso-called orderliness can be obtained. Then, second heat treatment ispreferably performed at a temperature higher than or equal to 400° C.and lower than or equal to 750° C. after the formation. The second heattreatment is performed in a nitrogen atmosphere, an oxygen atmosphere,or a mixed atmosphere of nitrogen and oxygen, whereby the density of thesecond crystalline oxide semiconductor layer 107 b can be increased andoxygen deficiency can be reduced. By the second heat treatment, crystalgrowth proceeds in the thickness direction with the use of the firstcrystalline oxide semiconductor layer 107 a as a nucleus, that is,crystal growth proceeds from the bottom to the inside; thus, the secondcrystalline oxide semiconductor layer 107 b is formed.

In a manner similar to the transistor 100, when the thickness of thecrystalline oxide semiconductor stack 120 is large, large currentbetween the source electrode and the drain electrode can be ensured inthe transistor 300.

The drain withstand voltage of the transistor 300 depends on thethickness of the crystalline oxide semiconductor stack 120. Therefore,in order to increase the drain withstand voltage, the thickness of thecrystalline oxide semiconductor stack 120 is preferably large and may beselected in accordance with the desired drain withstand voltage.

Therefore, considering the amount of the on-state current and the drainwithstand voltage, the thickness of the crystalline oxide semiconductorstack 120 is preferably greater than or equal to 0.1 μm and less than orequal to 50 μm, more preferably greater than or equal to 0.5 μm and lessthan or equal to 20 μm.

Further, the transistor 300 in which the crystalline oxide semiconductorstack 120 includes the channel region has orderliness in a directionalong the interface. Therefore, in the transistor 300, in the case wherecarriers flow along the interface of the crystalline oxide semiconductorstack 120, that is, in the case where carriers flow in a directionsubstantially parallel to the a-b plane, the crystalline oxidesemiconductor stack 120 does not block the flow. Therefore, degradationof the electrical characteristics of the transistor 300 is suppressedeven with light irradiation or application of a BT stress.

Without limitation to the two-layer structure in which the secondcrystalline oxide semiconductor layer 107 b is formed over the firstcrystalline oxide semiconductor layer 107 a, a stacked structureincluding three or more layers may be formed by repeatedly performing aprocess of deposition and heat treatment for forming a third crystallineoxide semiconductor layer after the second crystalline oxidesemiconductor layer 107 b is formed.

Steps for manufacturing the transistor 300 are described below. Thesteps for manufacturing the transistor 300 are the same as those formanufacturing the transistor 100 except a step for manufacturing thecrystalline oxide semiconductor stack 120. Therefore, the steps aredescribed with reference to FIGS. 8A to 8D.

In accordance with the manufacturing steps described in Embodiment 1,steps up to the formation of the gate insulating layer 105 areperformed, whereby a structure illustrated in FIG. 8A is obtained.

Next, a first oxide semiconductor film is formed over the gateinsulating layer 105. The thickness of the oxide semiconductor filmformed is smaller than the thickness of a second oxide semiconductorfilm which is to be the second crystalline oxide semiconductor layer 107b.

In this embodiment, the first oxide semiconductor film is formed to athickness of 100 nm in an oxygen atmosphere, an argon atmosphere, or anatmosphere including argon and oxygen under conditions where anIn—Ga—Zn-based metal oxide target (In₂O₃:Ga₂O₃:ZnO=1:1:2 [molar ratio])is used, the distance between the substrate and the target is 170 mm,the substrate temperature is 250° C., the pressure is 0.4 Pa, and thedirect current (DC) power is 0.5 kW.

Next, the atmosphere in a chamber in which the substrate is put is setto a nitrogen atmosphere or dry air, and first heat treatment isperformed. The temperature of the first heat treatment is higher than orequal to 400° C. and lower than or equal to 750° C. In addition, heatingtime of the first heat treatment is longer than or equal to 1 minute andshorter than or equal to 24 hours. By the first heat treatment, a firstcrystalline oxide semiconductor film is formed (see FIG. 8B). Details ofthe first heat treatment are described in Embodiment 1 and thus, areomitted here.

Next, a second oxide semiconductor film which has a large thickness thanthe first crystalline oxide semiconductor film is formed over the firstcrystalline oxide semiconductor film.

In this embodiment, the second oxide semiconductor film is formed to athickness of 400 nm in an oxygen atmosphere, an argon atmosphere, or anatmosphere including argon and oxygen under conditions where anIn—Ga—Zn-based metal oxide target (In₂O₃:Ga₂O₃:ZnO=1:1:2 [molar ratio])is used, the distance between the substrate and the target is 170 mm,the substrate temperature is 400° C., the pressure is 0.4 Pa, and thedirect current (DC) power is 0.5 kW.

Next, the atmosphere in a chamber in which the substrate is put is setto a nitrogen atmosphere or dry air, and second heat treatment isperformed. The temperature of the second heat treatment is higher thanor equal to 400° C. and lower than or equal to 750° C.

In addition, heating time of the second heat treatment is longer than orequal to 1 minute and shorter than or equal to 24 hours. By the secondheat treatment, the second crystalline oxide semiconductor film isformed (see FIG. 8C). Details of the second heat treatment for formingthe second crystalline oxide semiconductor film are similar to those ofEmbodiment 1. Note that in the drawings, an interface between the firstcrystalline oxide semiconductor film and the second crystalline oxidesemiconductor film is denoted by a dashed line for description of theoxide semiconductor stack; however, the interface is actually notdistinct and is illustrated for easy understanding.

When the first heat treatment and the second heat treatment areperformed at a temperature higher than 750° C., a crack (a crackextended in the thickness direction) is easily generated in the formedoxide semiconductor film due to shrink of the glass substrate.Accordingly, in the case where the temperatures of the first heattreatment and the second heat treatment and the substrate temperature atthe time of forming the oxide semiconductor film by a sputtering methodare set to temperatures lower than or equal to 750° C., preferably lowerthan or equal to 450° C., a highly reliable transistor can bemanufactured over a large area glass substrate.

It is preferable that steps from the formation of the gate insulatinglayer 105 to the second heat treatment be successively performed withoutexposure to the air. For example, a manufacturing apparatus whose topview is illustrated in FIG. 12 may be used. The manufacturing apparatusillustrated in FIG. 12 is the single wafer multi-chamber equipment,which includes three sputtering devices 10 a, 10 b, and 10 c, asubstrate supply chamber 11 provided with three cassette ports 14 forholding a process substrate, load lock chambers 12 a and 12 b, atransfer chamber 13, a substrate heating chamber 15, and the like. Notethat a transfer robot for transferring a substrate to be treated isprovided in each of the substrate supply chamber 11 and the transferchamber 13. The atmospheres of the sputtering devices 10 a, 10 b, and 10c, the transfer chamber 13, and the substrate heating chamber 15 arepreferably controlled so as to hardly contain hydrogen and moisture(i.e., as an inert atmosphere, a reduced pressure atmosphere, or a dryair atmosphere). For example, a preferable atmosphere is a dry nitrogenatmosphere in which the dew point of moisture is −40° C. or lower,preferably −50° C. or lower. An example of a procedure of themanufacturing steps with use of the manufacturing apparatus illustratedin FIG. 12 is as follows. A process substrate is transferred from thesubstrate supply chamber 11 to the substrate heating chamber 15 throughthe load lock chamber 12 a and the transfer chamber 13; moistureattached to the process substrate is removed by vacuum baking in thesubstrate heating chamber 15; the process substrate is transferred tothe sputtering device 10 c through the transfer chamber 13; and the gateinsulating layer 105 is deposited in the sputtering device 10 c. Then,the process substrate is transferred to the sputtering device 10 athrough the transfer chamber 13 without exposure to air, and the firstoxide semiconductor film is formed in the sputtering device 10 a. Then,the process substrate is transferred to the substrate heating chamber 15though the transfer chamber 13 without exposure to air and the firstheat treatment is performed. Then, the process temperature istransferred to the sputtering device 10 b through the transfer chamber13, and the second oxide semiconductor film is formed in the sputteringdevice 10 b. Then, the process substrate is transferred to the substrateheating chamber 15 through the transfer chamber 13, and the second heattreatment is performed. As described above, with use of themanufacturing apparatus illustrated in FIG. 12, the steps formanufacturing a transistor can proceed without exposure to air. Further,the sputtering device in the manufacturing apparatus in FIG. 12 canachieve a manufacturing process without exposure to air by changing asputtering target. For example, the following process can be performed.The substrate over which the gate insulating layer 105 is formed inadvance is placed in the cassette port 14, and the steps from theformation of the first oxide semiconductor film to the second heattreatment are performed without exposure to air, so that the firstcrystalline oxide semiconductor film and the second crystalline oxidesemiconductor film are formed. After that, in the sputtering device 10c, a conductive film to be the pair of second electrodes 109 a and 109 bcan be formed with use of a metal target over the second crystallineoxide semiconductor film, without exposure to air.

Next, a crystalline oxide semiconductor stack including the firstcrystalline oxide semiconductor film and the second crystalline oxidesemiconductor film is processed, so that the crystalline oxidesemiconductor stack 120 in which the first crystalline oxidesemiconductor layer 107 a and the second crystalline oxide semiconductorlayer 107 b are stacked is formed (see FIG. 8D).

A mask having a desired shape is formed over the crystalline oxidesemiconductor stack and then the crystalline oxide semiconductor stackis etched with the use of the mask to perform the processing of thecrystalline oxide semiconductor stack. The mask may be formed by amethod such as photolithography or an ink-jet method.

For the etching of the crystalline oxide semiconductor stack, either dryetching or wet etching may be employed. It is needless to say that bothof them may be employed in combination. Details of the dry etching andthe wet etching are similar to those of Embodiment 1.

The subsequent steps are the same as those of Embodiment 1. The pair ofsecond electrodes 109 a and 109 b are formed, and the insulating layer111 covering part of the crystalline oxide semiconductor stack 120 andthe pair of second electrodes 109 a and 109 b is formed. Further, thethird electrode 113 is formed in contact with the insulating layer 111so as to overlap with a channel formation region of the crystallineoxide semiconductor stack 120 (see FIG. 6A). Details of the steps aresimilar to those of Embodiment 1. Note that the step for forming thepair of second electrodes 109 a and 109 b is combined with a step forforming a wiring 110 (see FIG. 1A), and the step for forming the thirdelectrode 113 is combined with a step for forming a wiring 114 (see FIG.1A).

Through the above steps, a non-linear element with high withstandvoltage, low reverse saturation current, and high on-state current canbe obtained. Note that this embodiment can be implemented in combinationwith any of the structures described in other embodiments, asappropriate.

Embodiment 4

In this embodiment, a non-linear element whose structure is partlydifferent from that of the non-linear element described the aboveembodiment will be described. Note that a transistor is used as anexample also in this embodiment.

A transistor 400 described in this embodiment is a transistor obtainedin such a manner that the oxide semiconductor layer 107 of thetransistor 200 described in Embodiment 2 is replaced with a crystallineoxide semiconductor stack 120 including a first crystalline oxidesemiconductor layer 107 a and a second crystalline oxide semiconductorlayer 107 b. In short, a plan structure of the transistor 400 is similarto that of the transistor 200; therefore, FIG. 4A can be referred to forthe plan view of the transistor 400. FIG. 9A is a cross-sectional viewtaken along line E-F in the transistor 400 (see FIG. 4A). FIG. 9B is across-sectional view taken along line G-H in the transistor 400 (seeFIG. 4A). Note that in FIGS. 9A and 9B, the same reference numerals areused for the same parts as those in FIGS. 1A to 1C, and details of thesame reference numerals are omitted.

As illustrated in FIG. 9B, the transistor 400 is a dual-gate transistorand includes a base insulating layer 102, a first electrode 103, a gateinsulating layer 105, a crystalline oxide semiconductor stack 120, n⁺layers 117 a and 117 b, a pair of second electrodes 109 a and 109 b, aninsulating layer 111, and a third electrode 113, which are provided overa substrate 101.

The first electrode 103 is provided in contact with the base insulatinglayer 102. The gate insulating layer 105 is provided to cover the firstelectrode 103. The crystalline oxide semiconductor stack 120 is providedin contact with the gate insulating layer 105 to overlap with the firstelectrode 103. The n⁺ layers 117 a and 117 b cover the gate insulatinglayer 105 and end portions of the crystalline oxide semiconductor stack120. The pair of second electrodes 109 a and 109 b cover the endportions of the crystalline oxide semiconductor stack 120 and endportions of the n⁺ layers 117 a and 117 b. The insulating layer 111covers part of the crystalline oxide semiconductor stack 120 and thepair of second electrodes 109 a and 109 b. The third electrode 113 isprovided on and in contact with the insulating layer 111 and between thepair of second electrodes 109 a and 109 b.

The n⁺ layers 117 a and 117 b are formed between the crystalline oxidesemiconductor stack 120 and the pair of second electrodes 109 a and 109b, whereby contact resistance between the crystalline oxidesemiconductor stack 120 and the pair of second electrodes 109 a and 109b can be reduced. As a result, high on-state current can be obtained. Inaddition, when the n⁺ layers 117 a and 117 b are formed, parasiticcapacitance can be reduced and the amount of change in on-state current(Ion degradation) between before and after application of a negativegate stress in a BT test can be suppressed.

Although only one of the pair of second electrodes 109 a and 109 b isillustrated in FIG. 1C, an etching step is performed using the pair ofsecond electrodes 109 a and 109 b as masks to form the n⁺ layers 117 aand 117 b, and thus the n⁺ layers 117 a and 117 b are processed so thatthe end portions of the n⁺ layers 117 a and 117 b protrude from the pairof second electrodes 109 a and 109 b. Thus, the channel length of thetransistor 400 is determined by the distance between the n⁺ layer 117 aand the n⁺ layer 117 b. Although the pair of second electrodes 109 a and109 b face the crystalline oxide semiconductor stack 120 with the n⁺layers 117 a and 117 b provided therebetween, the pair of secondelectrodes 109 a and 109 b serve as a heat sink in a manner similar toother embodiments because the n⁺ layers 117 a and 117 b are not formedto be extremely thick and do not block the conduction of heat generatedin the crystalline oxide semiconductor stack 120. As a result, heatwhich is generated when high on-state current flows in the crystallineoxide semiconductor stack 120 can be dissipated to the outside, so thatdegradation of the transistor 400 due to heat generation can besuppressed.

The crystalline oxide semiconductor stack 120 has a stacked structure ofthe first crystalline oxide semiconductor layer 107 a and the secondcrystalline oxide semiconductor layer 107 b. Details of the firstcrystalline oxide semiconductor layer 107 a and the second crystallineoxide semiconductor layer 107 b are similar to those of Embodiment 3. Inthe transistor 400 described in this embodiment, at least part of thefirst crystalline oxide semiconductor layer 107 a and part of the secondcrystalline oxide semiconductor layer 107 b are crystallized and havec-axis alignment, and the crystalline oxide semiconductor stack 120 hasorderliness in a direction along the interface between the crystallineoxide semiconductor stack 120 and the gate insulating layer. Therefore,in the case where carriers flow along the interface, the crystallineoxide semiconductor stack 120 does not block the flow. Therefore,degradation of the electrical characteristics of the transistor 400 issuppressed even with light irradiation or application of a BT stress.

Steps for manufacturing the transistor 400 are described below. Thesteps for manufacturing the transistor 400 are the same as those formanufacturing the transistor 200 except a step for manufacturing thecrystalline oxide semiconductor stack 120. Therefore, the steps aredescribed with reference to FIGS. 5A to 5C and FIGS. 8A to 8D.

In accordance with the manufacturing steps described in Embodiment 1 andEmbodiment 3, steps up to the formation of the crystalline oxidesemiconductor stack 120 are performed, whereby a structure illustratedin FIG. 8D is obtained.

Next, the film 115 to be the n⁺ layers 117 a and 117 b is formed to athickness greater than or equal to 1 nm and less than or equal to 200 nmover the gate insulating layer 105 and the crystalline oxidesemiconductor stack 120 with the use of an In—Zn-based metal oxide, anIn—Sn-based metal oxide, or a single-component metal oxide containingindium or tin. The method for forming the film 115 is similar to that ofEmbodiment 2. In addition, SiO₂ may be contained in the above materialfor the n⁺ layers. In this embodiment, an In—Sn-based metal oxide filmcontaining SiO₂ is formed to a thickness of 100 nm.

Next, a conductive film for forming the pair of second electrodes 109 aand 109 b is formed over the film 115 which is to be the n⁺ layers (seeFIG. 5A). The conductive film is processed to form the pair of secondelectrodes 109 a and 109 b. Note that the step for forming the pair ofsecond electrodes 109 a and 109 b is combined with a step for forming awiring 110 (see FIG. 4A).

Then, the film 115 is processed using the pair of second electrodes 109a and 109 b as masks to form the n⁺ layers 117 a and 117 b. Through thisprocessing, the n⁺ layers 117 a and 117 b are formed so that the endportions thereof protrude from the pair of second electrodes 109 a and109 b (see FIG. 5B). Thus, the channel length of the transistor 400 isdetermined by the distance between the n⁺ layer 117 a and the n⁺ layer117 b. On the other hand, the channel length of the transistor 300described in Embodiment 3 is determined by the distance between the pairof second electrodes 109 a and 109 b. It is preferable that the taperangle of each of the end portions of the n⁺ layers 117 a and 117 b (anangle formed by the side surface of either the n⁺ layer 117 a or the n⁺layers 117 b and a planar surface of the substrate 101) be less than orequal to 30°.

The subsequent steps are the same as those of Embodiment 2. Theinsulating layer 111 covering part of the crystalline oxidesemiconductor stack 120 and the pair of second electrodes 109 a and 109b is formed. Further, the third electrode 113 is formed in contact withthe insulating layer 111 so as to overlap with a channel formationregion of the crystalline oxide semiconductor stack 120 (see FIG. 9A).Note that the step for forming the third electrode 113 is combined witha step for forming a wiring 114 (see FIG. 4A), and details of the stepsare the same as those of Embodiment 2.

Through the above steps, a non-linear element with high withstandvoltage, low reverse saturation current, and high on-state current canbe obtained. Note that this embodiment can be implemented in combinationwith any of the structures described in other embodiments, asappropriate.

Embodiment 5

In this embodiment, calculation results of on-state currents ofnon-linear elements will be described. Note that calculation isperformed on simplified structures of the non-linear elements. Further,Sentaurus Device manufactured by Synopsys Inc. is used in thecalculation.

First, a calculation result of a change in a drain current with respectto a change in a gate voltage is described.

FIG. 13A is a simplified view of a cross-sectional structure (Structure1) along the channel length direction in the transistor 200 described inEmbodiment 2 (a cross-sectional structure taken along line E-F in FIG.4A). FIG. 13B is a simplified view of a cross-sectional structure takenalong line G-H in FIG. 4A. FIG. 13C is a simplified view of across-sectional structure along a direction perpendicular to the lineE-F in a channel formation region of the transistor 200. Note thatcomponents of FIGS. 13A to 13C corresponding to those of FIGS. 4A to 4Care denoted by the same numerals as those of FIGS. 4A to 4C.

Parameters reflected on the calculation result of on-state current inthe structure of FIGS. 13A to 13C are as follows:

1. Channel length L1: 10 μm

2. Length L2 of the pair of second electrodes 109 a and 109 b: 5 μm

3. Thickness T_(os) of the oxide semiconductor layer 107: 10 μm

4. Thickness T_(G) of the gate insulating layer 105 and the thicknessT_(BG) of the insulating layer 111: 0.2 μm

5. Channel width W1: 100 μm

6. Width W2 of the pair of second electrodes 109 a and 109 b: 5 μm

7. Work function ϕM of tungsten used for the first electrode 103: 4.9 eV

8. Work function ϕM of titanium used for the pair of second electrodes109 a and 109 b: 4.0 eV

9. Work function ϕM of molybdenum used for the third electrode 113: 4.8eV

10. Band gap Eg, electron affinity χ, relative permittivity, andelectron mobility of In—Ga—Zn—O-based metal oxide used for the oxidesemiconductor layer 107: 3.15 eV, 4.3 eV, 15, and 10 cm²/Vs

11. Relative permittivity of silicon oxynitride used for the gateinsulating layer 105: 4.1

12. Relative permittivity of silicon oxide used for the insulating layer111: 3.8

Note that the calculation is performed on the assumption that the firstelectrode 103, the pair of second electrodes 109 a and 109 b, the thirdelectrode 113, and the n⁺ layers 117 a and 117 b have the same potentialregardless of their thicknesses, and thus, the calculation results donot reflect the thicknesses.

FIG. 14 shows the calculation result of the drain current (Id) where thedrain voltage is 15 V and the gate voltage varies from 0 V to 20 V. Asfound from FIG. 14, the non-linear element in which the end portions ofthe oxide semiconductor layer 107 are covered with the n⁺ layers 117 aand 117 b and the pair of second electrodes 109 a and 109 b can havehigh on-state current.

Next, a calculation result of a change in the drain current with respectto a change in the drain voltage in Structure 1 is described.Comparative examples are Structure 2 to Structure 4, which are describedbelow.

Structure 2 is a structure of the transistor 200 (see FIGS. 15A to 15C),in which the pair of second electrodes 109 a and 109 b, the n⁺ layers117 a and 117 b, and the third electrode 113 are not in contact with theside surfaces of the oxide semiconductor layer 107. As for Structure 2,FIG. 15A is a view simplified in the same manner as FIG. 13A. FIG. 15Bis a view simplified in the same manner as FIG. 13B. FIG. 15Ccorresponds to the structure of FIG. 13C.

Structure 3 is a structure (see FIGS. 16A to 16C) in which the pair ofsecond electrodes 109 a and 109 b and the n⁺ layers 117 a and 117 b arein contact with the side surfaces of the oxide semiconductor layer 107,and the third electrode 113 is not in contact with the side surfaces ofthe oxide semiconductor layer 107. As for Structure 3, FIG. 16A is aview simplified in the same manner as FIG. 13A. FIG. 16B is a viewsimplified in the same manner as FIG. 13B. FIG. 16C corresponds to thestructure of FIG. 13C.

Structure 4 is a structure (see FIGS. 17A to 17C) in which the pair ofsecond electrodes 109 a and 109 b and the n⁺ layers 117 a and 117 b arenot in contact with the side surfaces of the oxide semiconductor layer107, and the third electrode 113 is in contact with the side surfaces ofthe oxide semiconductor layer 107. As for Structure 4, FIG. 17A is aview simplified in the same manner as FIG. 13A. FIG. 17B is a viewsimplified in the same manner as FIG. 13B. FIG. 17C is a view simplifiedin the same manner as FIG. 13C.

Parameters of Structure 2 to Structure 4 which are reflected incalculation results are the same as those of Structure 1. Note that thecalculation is performed on the assumption that the first electrode 103,the pair of second electrodes 109 a and 109 b, the third electrode 113,and the n⁺ layers 117 a and 117 b have the same potential regardless oftheir thicknesses, and thus, the calculation results do not reflect thethicknesses.

The calculation results of the on-state currents of Structure 1 toStructure 4 are shown in FIG. 18. FIG. 18 shows the calculation resultsof drain currents (Id) corresponding to drain voltages, where the gatevoltage (Vg) is 10 V and the drain voltages (Vd) vary from 0 V to 20 V.

As shown in FIG. 18, the drain current of Structure 1 is higher than anyof the drain currents of Structure 2 to Structure 4. In other words, astructure like Structure 1 in which the pair of second electrodes 109 aand 109 b, the n⁺ layers 117 a and 117 b, and the third electrode 113are in contact with the side surfaces of the oxide semiconductor layer107 is employed, carriers can be efficiently injected into the oxidesemiconductor layer 107 that includes the channel formation region andhigh on-state current can be obtained, which is favorable to anon-linear element for large current application.

This embodiment can be implemented in appropriate combination with anyof the structures described in the other embodiments.

Embodiment 6

In this embodiment, examples of a power diode and a rectifier with theuse of a non-linear element that is one embodiment of the presentinvention will be described with reference to FIG. 10A1 to 10C2 andFIGS. 11A and 11B.

FIG. 10A1 illustrates an example of a structure of a power diode that isone embodiment of the present invention. In a power diode illustrated inFIG. 10A1, a plurality of diodes are connected in series.

FIG. 10B1 illustrates an example of a structure of a rectifier that isone embodiment of the present invention. The rectifier illustrated inFIG. 10B 1 is a half-wave rectifier including two diodes. An anode of afirst diode is connected to a lower potential side reference potential(preferably, a ground potential). A cathode of the first diode isconnected to an input portion and an anode of a second diode. A cathodeof the second diode is connected to an output portion.

FIG. 10C1 illustrates an example of a structure of a rectifier that isone embodiment of the present invention. The rectifier illustrated inFIG. 10C1 is a full-wave rectifier including four diodes. The fourdiodes are referred to as first to fourth diodes clockwise from thediode on the upper left. Anodes of the first diode and the fourth diodeare connected to a reference potential (preferably a ground potential)on the lower potential side. A cathode of the first diode and an anodeof the second diode are connected to a first input portion. An anode ofthe third diode and a cathode of the fourth diode are connected to asecond input portion. A cathode of the second diode and a cathode of thethird diode are connected to an output portion.

As the diodes used in the power diode, the half-wave rectifier, and thefull-wave rectifier, non-linear elements described in the aboveembodiment, in each of which one of the pair of second electrodes 109 aand 109 b which functions as the source electrode or the drain electrodeis electrically connected (diode-connected) to the first electrode 103functioning as the gate electrode can be used (see FIGS. 11A and 11B).

In the case where the non-linear element described in the aboveembodiment is an n-type non-linear element, electrodes with diodeconnection are an anode and an electrode without diode connection is acathode.

FIG. 11A is a plan view of the diode-connected non-linear element. FIG.11B is a cross-sectional view taken along line I-J in FIG. 11A. Asillustrated in FIG. 11B, a wiring 110 including the pair of secondelectrodes 109 a and 109 b is electrically connected to a wiring 104including the first electrode 103 through an opening portion 150. Notethat although the transistor 100 described in Embodiment 1 is used inthis embodiment, the transistor of any of Embodiment 2 to Embodiment 4can also have a diode connection.

The power diode of FIG. 10A1 can have a structure illustrated in FIG.10A2 with the use of diode-connected transistors. The half-waverectifier of FIG. 10B1 can have a structure illustrated in FIG. 10B2with the use of diode-connected transistors. The full-wave rectifier ofFIG. 10C1 can have a structure illustrated in FIG. 10C2 with the use ofdiode-connected transistors.

In FIG. 10A2, the transistors included in the power diode, which aredual-gate transistors, each include a third electrode 113 (including awiring 114) (see FIGS. 11A and 11B). The third electrodes 113 to whichcontrol signals G1 to G5 are supplied control threshold voltages of therespective diode-connected transistors. Also in FIGS. 10B2 and 10C2,since every transistor includes the third electrode 113, the thresholdvoltages of the diode-connected transistors can be controlled by thecontrol signals G1 to G4. For example, in view of reliability asdescribed in the above embodiment, the electrical characteristics of atransistor including an oxide semiconductor vary by irradiation withvisible light and ultraviolet light or application of heat or anelectric field. For example, a transistor becomes normally-on. Furtherin the case where the half-wave rectifier and the full-wave rectifierare formed using n-channel transistors, when the n-channel transistorsbecome normally-on, current flows in the half-wave rectifier and thefull-wave rectifier even when a reverse bias is applied thereto, wherebya normal rectification action cannot be obtained. Thus, negativepotential is applied to the third electrodes 113 functioning as backgate electrodes of the transistors included in the half-wave rectifierand the full-wave rectifier, which prevents the transistors frombecoming normally-on and reduces reverse current, so that a favorablerectification action can be obtained.

Note that in this embodiment, the third electrodes 113 of thetransistors included in the power diode and the rectifier are suppliedwith respective control signals; however, the third electrodes 113 maybe connected to each other and the transistors included in the powerdiode and the rectifier may be supplied with the same signals. Further,in the drawings, “OS” written in the vicinity of a circuit symbol of thetransistor included in the power diode and the rectifier indicates thatthe transistor includes an oxide semiconductor layer.

Further, an oxide semiconductor can be used in the transistors includedin the power diode and the rectifier described in this embodiment.Therefore, according to any of the above embodiments, a power diode anda rectifier having excellent drain withstand voltage and high draincurrent can be obtained.

Through the above steps, a non-linear element with characteristics suchas high withstand voltage and low reverse saturation current, which canhave high on-state current can be obtained. Note that this embodimentcan be implemented in combination with any of the structures describedin other embodiments, as appropriate.

This application is based on Japanese Patent Application serial no.2010-204693 filed with Japan Patent Office on Sep. 13, 2010, the entirecontents of which are hereby incorporated by reference.

1. (canceled)
 2. A semiconductor device comprising: a substrate; a firstelectrode over the substrate; a gate insulating layer over the firstelectrode; an oxide semiconductor layer overlapping with the firstelectrode with the gate insulating layer therebetween; a first layercomprising indium and zinc over and in contact with the oxidesemiconductor layer; a source electrode comprising a region in contactwith the first layer; a second layer comprising indium and zinc over andin contact with the oxide semiconductor layer; a drain electrodecomprising a region in contact with the second layer; an insulatinglayer over the source electrode and the drain electrode; and a secondelectrode over the insulating layer, wherein the second electrodecomprises a region overlapping with the oxide semiconductor layer,wherein the second electrode comprises a region overlapping with thesource electrode and a region overlapping with the drain electrode,wherein the first layer comprises a region extending beyond an endportion of the source electrode, wherein the second layer comprises aregion extending beyond an end portion of the drain electrode, whereinan angle between a side surface of the first layer and a top surface ofthe substrate is less than or equal to 30° in a cross-sectional viewalong a channel length direction, wherein an angle between a sidesurface of the second layer and the top surface of the substrate is lessthan or equal to 30° in a cross-sectional view along the channel lengthdirection, wherein the oxide semiconductor layer comprises indium,gallium, and zinc, wherein a composition of the first layer is differentfrom a composition of the oxide semiconductor layer, and wherein acomposition of the second layer is different from a composition of theoxide semiconductor layer.
 3. The semiconductor device according toclaim 2, wherein the oxide semiconductor layer comprises a region havingc-axis alignment along a direction perpendicular to a top surface of theoxide semiconductor layer.
 4. A semiconductor device comprising: asubstrate; a first electrode over the substrate; a gate insulating layerover the first electrode; an oxide semiconductor layer overlapping withthe first electrode with the gate insulating layer therebetween; a firstlayer comprising In—Zn-based oxide over and in contact with the oxidesemiconductor layer; a source electrode comprising a region in contactwith the first layer; a second layer comprising In—Zn-based oxide overand in contact with the oxide semiconductor layer; a drain electrodecomprising a region in contact with the second layer; an insulatinglayer over the source electrode and the drain electrode; and a secondelectrode over the insulating layer, wherein the second electrodecomprises a region overlapping with the oxide semiconductor layer,wherein the second electrode comprises a region overlapping with thesource electrode and a region overlapping with the drain electrode,wherein the first layer comprises a region extending beyond an endportion of the source electrode, wherein the second layer comprises aregion extending beyond an end portion of the drain electrode, whereinan angle between a side surface of the first layer and a top surface ofthe substrate is less than or equal to 30° in a cross-sectional viewalong a channel length direction, wherein an angle between a sidesurface of the second layer and the top surface of the substrate is lessthan or equal to 30° in a cross-sectional view along the channel lengthdirection, wherein the oxide semiconductor layer comprises indium,gallium, and zinc, wherein a composition of the first layer is differentfrom a composition of the oxide semiconductor layer, and wherein acomposition of the second layer is different from a composition of theoxide semiconductor layer.
 5. The semiconductor device according toclaim 4, wherein the oxide semiconductor layer comprises a region havingc-axis alignment along a direction perpendicular to a top surface of theoxide semiconductor layer.